As Phil said, there is an answer: ask your printer.
If, for example, it's being printed on 11x17 paper and you want full bleeds (bleeds on all sides), it'll definitely have to be trimmed down to something smaller than 8.5x11. If, on the other hand, it's being printed on a large press that's printing multi-up then 8.5x11 should be fine, or even 9x12.
If the book is going to be saddle stitched (staples on a fold) then you'll want to learn about page creep (the inner pages will be trimmed more than the outer pages). If it's going to be perfect bound (flat spine) then you'll need to worry about having a large enough gutter to keep people from having to break the spine of the book in order to read everything (but not so large that you're wasting space).
Seriously, talk to your printer. There are far, far too many variables to provide a meaningful answer.