Writers are always wondering How Long their books should be. Really it's probably a little flexible based on the writing but there is one definitive source for words of wisdom on word counts: and here it is.
If you want to be safe, you should stay in the "acceptable" range. I know it's easy to fall outside the norm but don't give agents and editors a good reason to toss your pages in the trash. Edit, edit, edit until you fit the mold they're looking for.
Now, Happy Writing!