Does the size of your print book matter? You bet it does. If you have been writing a book that is important to you, you probably did not start out by saying, "OK, I'm going to write a book that measures 8" x 5." You may not have thought about how long the book would turn out to be. While you are writing, ideas, plots, and characters are your main concern. But when you finish the writing and begin the process of identifying a book creation service, length and physical size will become important factors. If you want your finished product to have a professional appearance, you'd better know what you are doing.
Here are some guidelines to get you started:
Mass-Market Books: Most books in this category are bodice-rippers, teen romance, or hot and heavy he-man stories -- the kind of books that are almost indistinguishable from one another except for the cover images. You find these on racks in grocery markets, airports, drugstores, and convenience stores. Those racks only hold one size--4-1/4" by 7". so if that kind of impulse purchase is your goal, your book will have to fit the shelf exactly. But if you are aiming to produce a serious novel, avoid this size.
Novels: There is no one standard for a novel. At the small end, 5-1/4" x 8" provides an intimate reading experience. The most common size is 5-1/2" x 8-1/2", which has a nicely-balanced feel to it. And if your novel is long -- say, over 500 pages-- you may want to move up to a 6" x 9" size to make it feel less bulky. All of those sizes will do well in either hard cover or perfect-bound (soft) cover.
Trade Paperbacks: This is the serious fiction/non-fiction category into which most self-published books fall. These books are roughly the same size as the novels described above, but they can grow a bit larger for business books or textbooks, particularly if the book will have illustrations or graphics. They start at 5-1/2" x 8-1/2" and can go up to 7" x 10" depending on content. Note that the ideal ratio for a book is 2:3, so a 6" x 9" is particularly appealing.
Manuals, Cookbooks, and Workbooks: These are larger books, either 8" x 10" or 8-1/2" x 11" and often bound with a spiral coil to allow the book to lie flat for easy note-taking. This size also lends itself to a two-column format for easier reading, but many self-publishing or print-on-demand companies are not yet equipped to produce the larger sizes.
How important is it that you make the right choice? Crucial! There may be situations in which size does not matter, but book printing is not one of them. My own experience proved that. Right after I retired, I wrote what I wanted to be a popular history of a little-known battle in the Civil War. I visualized a general reading audience of Civil War enthusiasts, reenactors, and lovers of history.
However, my publisher, a small and relatively inexperienced private company, planned to sell the book in museums and Civil War battlefield shops, and they chose a size just a little bigger than usually found in that type of retail outlet -- 7-1/2" x 10" -- which they thought would make the book stand out from the rest. Almost immediately, they discovered that the text was hard to follow at that width, so they decided to use a two-column format. The result? A book that looked like a junior high school textbook being sold (or in this case NOT SOLD) in shops aiming at a general adult audience.The book did not sell until they put it on Kindle, which got rid of the awkward sizing problem.