A detailed article from The Book Designer on considerations for your self-published print book, including the different formats and sizes offered by the various online printing services and distributors:
Some pricing on digital books is in a range of sizes rather than having a different price for every different size, but that only helps a bit.
If you plan to print offset, you’ll need to specify the exact size in your request for an estimate. So one way or the other, it’s good to figure out near the beginning of your planning.
I can't really add much to this. If you are looking into self-publishing in print I'd click through right now.
Full article here: http://www.thebookdesigner.com/book-sizes/