If you are a writer, these and other questions just like them have undoubtedly crossed your mind at one time or another. As for me, they are on my mind all the time, and I don't know about you, but I can't seem to get a strait answer. However, I have recently found that the reason I can't get a strait answer is that simply put - there isn't one. Yes, there are guidelines that writers should try to keep to - especially if you are a new writer looking to publish for the first time - those being 80,000 to 120,000 words. Guidelines yes, but let us not forget the gray area. YA is typically shorter, while Fantasy can be longer, and so on. There is no set in stone rule ~ so what am I trying to say then? Simply this: Write the best story you can without regard to word count. The biggest mistake some writers make is worrying about word count before they have finished - or in some cases even begun - their book. Don't do it. You may end up throwing junk in the story that should be there just to fill space or cutting something important to save time. Just write the story you want to write, and worry later.
For those of you who are still looking for numbers, here is a chart I found that give some of the standard expectations:
Juvenile picture book
Young adult book
(4-16 lines preferred)
Hope that helps, though I pretty sure it doesn't, as it did little to ease my mind... but hey, I tried.
Okay, how bout this... *Ahem* You... you there... reading right now... yes, you - your book should be 92,573 words. There you go, problem solved.
Now get writing! :)