Size Matters: I like to meet with (potential) customers before they sit down to write. When I don't, they often show up with a manuscript that is the wrong size. Yes, there is a wrong size. While it is true that sometimes a book needs to be as long as it needs to be, as some might say, more often authors deliver the wrong size manuscript because they failed to plan.
What is the wrong size? It's a size that is not right for your readers, your purpose, or your budget. (For simplicity sake, I'm your project does not involve a color interior. That's an added complication.)
Your readers: There is an optimum size for your readers, whether they are children, teens, or adults. If there are too many words, they might not pick up your book at all. If they do, they might not finish it. If there are not enough words, your readers might not take your work seriously.
Your purpose: Here's where trim size gets important. If your book consists entirely of words, you can plan for a smaller trim size than you would want if your work was heavily dependent upon images.
Your budget: I put this third, but it's essential for everyone who is not independently wealthy.The optimum size is the one that enables you to make money on every sale. Your unit printing cost, which in the print-on-demand world is determined by trim size and number of pages, must be such that you can comfortably set your retail price at five times your unit printing cost. If your book is too long, your unit printing cost will be too high and you won't be able to set a high enough retail price to do business.
For a ballpark idea of what I'm talking about, let's say you are planning a non-fiction words-only book for adults. Therefore, all things being equal, I would advise you to produce a manuscript of approximately 50,000 words, which will fit into a 160-page book in a small format (5.5"x8.5", typically). This book will cost you less than $4.00, including shipping at CreateSpace, a unit cost that will enable you to set a retail price of $20.00.
As I said, this is ballpark. The small format 160-page model is not right for every book, but the principle is. There is an optimum size for your book. With a little planning, you can come in right on the money. The Publishing Pro.