Somewhere along the line, I read that a YA book by a first-time author should never be more than 80,000 words. I think it had something to do with the cost of publishing verses the risk of the unknown, as well as the (apparent) ideal length an average young-adult wants to read.
So, always keen to take on good advice, I've tried to keep to this length in all my writing, and have usually managed to come in almost exactly on target (by dumb luck rather than careful planning, I can promise).
But then recently I've read two books by first-time authors that are more in the region of 200,000 words. The first is Wounded Guardian, by Duncan Lay (well worth a visit to his blog, he gives some great advice), and the second is book 1 of the Monster Blood Tattoo series. Perhaps the 80K rule is one of those things that generally applies, but can sometimes be broken.
For me, though, I've always been comfortable with the 80,000 word target. I think if I tried to write something bigger than that, I'd probably end up fluffing it out with unnecessary detail and would only bore the daylights out of the reader, rather than adding anything of value. Perhaps that's just my style, or perhaps that's the length of book that I prefer as a reader (not to take away from either of the excellent titles referenced above).
Or perhaps it's the nightmare of having to edit a 200,000 word manuscript. I'm finding 80,000 words challenging enough as it is!