The short answer is that it is completely up to you. The longer answer is, if you want people to be able to read, understand, and absorb what you wrote, it matters very much how you present it to them.
I'll note that this essay here specifically focuses on a full length novel. I cover novellas and short stories in other areas of this site.
I've taken numerous courses on how people learn information. The courses were in the context of designing a high quality training program, but the concept is the same. You want to impart knowledge to people, and you want those people to be able to absorb and retain that knowledge. After all, if by the time they get to page 50 of your 500 page novel they are already confused about the characters and plots, they are likely to just close the book and give up. You absolutely need them to learn and understand the world you are presenting to them. It is your responsibility, as the author, to do this as well as possible. The reader, after all, has little say in this. All they can do is read what you present to them.
So the first place to start is with industry standards. What have others found, after decades of research, is the best length of material that a reader can absorb and juggle at once?
While we readers sometimes try to think in "page count", it's important to remember that books come in all shapes and sizes. Some have large fonts. Some have tiny fonts. With ebooks, the reader can change the font on the fly as eyes get tired. So really a "page size" means nothing at all. You need to reorient your thoughts into word count.
Let's take just one genre and look at word counts.
SCIENCE FICTION / FANTASY:
For a SFFA Nebula award, a novel has to have at least 40,000 words to qualify. Otherwise it's not a full length novel.
Frankenstein: 75,000 words
Harry Potter and the Sorcerer's Stone: 77,000 words
The Hobbit: 95,000 words
A Game of Thrones: 298,000 words
The Mists of Avalon: 416,000 words
So certainly part of what we see here is that a book CAN be successful at varying lengths. Even though the industry standard is around 100,000 words, books can do well if they have less. And people will hang in there if they are longer. But it's also fair to say that Marion Zimmer Bradley didn't tackle her 416,000 word tome until decades into her writing career. By then she had built up a large following, perfected her craft, and proven over and over again her talent with words. So when she then presented her epic look into the King Arthur saga, following generations of women, she knew what she was doing. J.K. Rowling didn't create her longest work, Harry Potter and the Order of the Phoenix at 257,000 words, until she was five books into the series. She made sure her fans were ready for it and that her writing skills had been honed and perfected through feedback on previous releases. George R. R. Martin had written many stories and short stories before turning to his Game of Thrones series.
There are reasons that most publishers and readers want books that are about 100,000 words in length and balk at entries that are shorter or longer.
First, the physical issue. If a book is too short, it feels flimsy in the hand. It's an emotional reaction. Think about going down the rows at a bookstore. A thin book will barely be spotted and will seem as if it doesn't offer much. On the other hand, if a book is too massive, it seems like a chore to hold it and read it. Hefting the massive tome of War and Peace in one's hands while paging through it can literally tire out the arms. Readers would much rather have multiple smaller books to hold up and carry around with them.
You might say that you only want to do an ebook version. But that limits yourself greatly. Are you really saying that your vision of this book is that nobody would ever want it in print form? That you want no publisher to ever show interest in it? Your dreams for this book should be that it takes over the world and everyone adores it. You want to make sure you do everything you can to make the book ready for that success. You shouldn't deliberately do things which would harm its chances.
Next, there is the endurance issue. When you talk with people about their efforts to read War and Peace, a common theme you hear from people is that they had to take multiple tries at it. That memorizing all of the confusing names, relationships, plot lines, and interwoven situations was baffling. They read it once and gave up. The read it a second time and, now knowing some of the basics, they got further before they got stuck. Maybe on the third or fourth time they knew enough of the background and characters that they could make it all the way through.
It's easy for an author to say "Oh, but an intelligent person would clearly be able to keep track of all my characters and events." That's not true at all. Even the most brilliant of memorists (and I do know a few people who do memorization "professionally") get worn down by having to keep track of billions of characters and situations. It simply isn't fun. Sure, the author can keep track of those things. It's their world. They know and care for these characters. The reader is new to this world! It is tedious to have to keep track of all of that.
That is why in training it is so key to take breaks and allow space for the trainee to absorb and process the information. They need regular breaks to internalize the information, see how it fits together, and how they feel about it.
That is exactly what having separate books in a series allows the reader to do. When the reader reads the first book - let's say the first Harry Potter book, they are given a "digestible chunk." A normal 77,000 words. They get to know the main characters. They form relationships with them. They understand their motivations. They take a trip with those characters and it winds up with an ending. The reader is pleased. They built that relationship, they came to know them, and the characters have a meaning. Now when they go to the next book that foundation is set. They are now familiar with the environment and characters. Now the book can get denser and more complex, because the foundation layer is laid. The reader is building on what they know and can delve more deeply.
With The Mists of Avalon, Bradley took a different approach. She wrote about a very well known story - the Arthurian Saga. So readers already knew the characters, the plot, and the environment. All they had to do is learn about her specific versions of the characters. That meant they could settle into the world easily and be drawn into the epic plot without the mental overload. They had that strong foundation. And I'll note that she broke her epic into four sub-books. She had Book One: Mistress of Magic, Book Two: The High Queen, Book Three: The King Stag, and Book Four: The Prisoner in the Oak. These easily could have released as separate physical books, given the length of the overall tome. Each sub-book is a "proper healthy" 100,000 words.
The third point is much more individual. It speaks to why most well written books fall within that 100,000 word range.
If a book is too short, it often does not have enough character or plot development. Characters are shallow and do not have much life to them. The plot is weak and doesn't have much interest to it. If a writer presents a full story in only 50,000 words, how much could they really be saying? That is why most publishers have a minimum of 80,000 words for submissions. They want to know this is a real, fully developed story they are going to look at. They don't want a story about a walk from one end of a village to another. They want a story that will engage a reader's attention and make them care about what happens. You need a certain amount of words to pull that off.
Certainly we have all seen amazingly written short stories. But that is quite a talent, to pull off the full plot and emotional connection in a limited number of words. It's probably not something to attempt on your first book or two.
If the book is too long, often it is quite bloated with excess detail. It is key for a story to keep the reader actively engaged, and to continually move the plot forward. There should not be page after page of the heroine staring dreamily out the window admiring the grass. Yes, maybe it happened - but do we care?
For example, in Lord of the Rings - Fellowship of the Ring we get 177,000 words - but it's broken up into two "books" to give readers that mental break. So each book is about 90,000 words long. "Book 1" (the first half of the physical book) ends when Frodo falls by the river, seriously injured, having barely escaped from the black riders and reaching Rivendell. "Book 2" (the second half of the same physical book) begins several weeks later when he finally comes to after being ill for several weeks. We don't hear about those intervening weeks - and we don't need to! They have nothing to do with the furtherance of the plot. The break gives us a mental pause where we can recoup from what came before, absorb and process that knowledge, and then prepare for the new challenges which lay ahead. And remember, this is just within the first book. Tolkien broke his saga up into three separate books in order to give us those breaks and pauses.
There have been many times in writing my books that I finally cut out long passages of the story, because they weren't critical to the plot. Readers don't want to plod along through less-than-important details. They want to see growth in their characters and new details about what is happening. In the first book I released, Seeking the Truth, I was advised by several of my readers to trim out excess material in the first few chapters of the book. I kept it in because I felt it was important to the story. But once the book was released, I heard from numerous reviewers out in the "real world" that they felt the story dragged at the beginning. Those details that I loved just weren't critical to the story's progression. Yes, perhaps they were interesting, but they didn't let the story progress in a meaningful way.
This is one of the hardest things for a writer to do - to remove words. We labor over our words and become emotionally attached to them. But it's not necessary to give every detail about a ride through a forest. It's not necessary to give every detail about a recovery from a battle. The reader knows the character isn't going to die. What the reader wants to know is what happens next - what is going on in the "real world".
Now, this being said, there are times that I have detailed the day-by-day recovery process. In Believing Your Eyes, a key part of how the hero and heroine build their relationship is while she is healing from a serious wound. So it is the way he tends to her, and the conversations they have, that draws them close together. But in the Lord of the Rings example, we already had relationships with the characters. We already knew how they interacted with each other. So further interaction while Frodo healed would have been gratuitous. What we the readers care about at that point is what is going to happen next with the ring. So Tolkien moves right to that point, to keep us engaged.
In the end, of course, you are the author. You have the final say about what you include in your book. You have the final word about how much detail you stuff into it, or how sparsely you write. But if you're hoping to have the ability to work with publishers, and want to be able to effectively reach the minds of your readers, it's wise to at least consider why the industry standards exist. They aren't arbitrary. They are what decades of experience have found to be most effective at getting a message through to a reader's mind.
Here are the book lengths for my medieval romance novels. This genre's publishers typically expect 65,000 - 80,000 words per novel. This length is what romance novel publishers find is the right balance between "I didn't hear enough about these characters" and "get to the point already, how long will this go on?" :). Pride and Prejudice at 120,000 words is about the longest most readers will wait before they get their "happy ending". Sense and Sensibility was 118,000 words. Wuthering Heights was 108,000 words.
Seeking the Truth - 115,000 words (this is the one readers said needed trimming)
Knowing Yourself - 71,000 words
A Sense of Duty - 87,000 words
Creating Memories - 88,000 words
Looking Back - 73,000 words
Badge of Honor - 99,000 words
Lady in Red - 68,000 words
Finding Peace - 67,000 words
Believing Your Eyes - 110,000 words
Trusting in Faith - 100,000 words
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