The biggest reason to worry about the size of your book is printing costs. It costs more to print a 500 page book than it does a 200 page one. Yes, it’s possible to decrease the number of pages by printing on a larger page—a 250 page book with 100,000 words will fit into fewer than 120 pages if you use 8 ½ x 11 paper instead of the standard 6×9…but then your book won’t fit on the shelf with all the others in the same genre. You do want to stand out, but not for that reason.
The book needs to be long enough to tell the whole story, but short enough to keep the reader’s attention. The actual length will depend on what genre you’re writing in. (If you can’t identify your genre, then your writing is too loose—keep the focus!) Certain genres tend to run longer than others do (history), and some run shorter (mystery), so check out what a common length is for the one you’re writing.
If you’re too long
When you’ve written more than is needed, the first thing to consider is if you put things in the reader doesn’t need to know…or should already know from a previous section—don’t explain too much. Let the reader figure things out for himself. Info dumps give the reader an opportunity to set the book down—and not pick it up again. Either the reader already knows the historical background of all the characters…or it isn’t necessary for the particular story you’re telling.
Another problem is giving too much detail when you shift scenes. We don’t need to see each and every character get up, walk across the room, and exit…only to enter the next scene the same way. Just end each scene with a quick cut and start the next with a short intro of who, where, and what.
I believe more in the scissors than I do in the pencil.—Truman Capote
A rather irksome tendency of some writers is “As You Know, Bob”. AYKB is where characters discuss things they already know…just so the reader can hear about it. How many times do you stand on the corner watching cars drive by and comment on how interesting it is that they all have four wheels? Not very exciting…either for you or the reader. Leave it out.
If you’re short
This isn’t going to be a problem for most writers, but if you find yourself in this situation, look for sections with little dialogue or action…places where you are telling the story instead of showing it. Expand that exposition into a full scene. Consider adding in a subplot in Act 2…pull in a secondary character and give him something to do.
Some books end too abruptly…almost as if the author gave up and just quit. Make certain you tie up all the loose ends (or imply that they will be taken care of in the next book!).