First, people have an impression of a book based on its size. If it's a standard small, paperback size, they assume it's a casual, enjoyable read. This is the size that most mystery books and romance novels come in.
If they pick up a hefty tome with hard covers, they get a different impression. This has more serious content, that they will have to pay close attention to.
Comic books tend to have a certain size, that allows the reader to enjoy the graphic panels on larger sized pieces of paper.
The standard letter size, 8.5 x 11, is often used for textbooks and instructional manuals.
It's also important to keep in mind the size of your book has a big impact on the cost - and therefore how many people will be able to afford to buy it! If your book has only 100 pages in a large size, it could be inexpensive - but if it runs to 400 pages when you squash it down into a smaller size, the per-page charge could cause your book to cost three times as much. That might change it from a book that people love to buy to one they think is overpriced.
Here is a chart of standard sizes that most publishers offer.
4.18" x 6.88", 4.25" x 6.875" - Small Paperback
5" x 8", 5.25" x 8", 5.5" x 8.5"- Standard Paperback
6" x 9", 7" x 10", 8" x 10" - Novel
8.5" x 11" - U.S. Letter
7.5" x 7.5" - Square
8.5" x 8.5" - Larger Square
6.625" x 10.25" - Comic Book
9" x 7" - Landscape
6.14" x 9.21" - Royal
7.5" x 9.25" - Technical Manual
7.44" x 9.68" - Crown Quarto
8.27" x 11.69" - A4
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