I just Googled it. BookEnds has a great post here. Colleen Lindsay has laid it out in handy bullet points here. Even Wikipedia puts its two cents in here, by way of a chart to show the word counts for novels, short stories, and novellas.
When reading queries, agents do discriminate based on word length. Even if your query is pretty good, if you're pitching anything over 120k, I'm going to reject it. Only in fantasy or some sci-fi will I look at anything over 100k. On the other side of the coin, I will also reject (most) anything under 60k (including YA--I've mentioned in a previous post here that I prefer longer books; just don't get too long).
If you are a debut author, stick to the guidelines. When you're famous you can do whatever you want.
Based on genres I represent and what I'm looking for, I give you the bullet-pointed list of word counts:
- YA contemporary: 60k-90k
- YA fantasy/para/sci-fi: 70K-100k
- Adult para/UF/sci-fi/fantasy: 70k-100k
Before you query or pitch, get your word count to a desirable length. Never say in a query or pitch that "I know it's a bit long/short, but I am open to suggestions on how to lengthen/shorten." This your job as the writer--deliver the best product possible. More than once I was being pitched a novel and when asked the word length the writer would reply with the word count while wincing. If you wince when you tell people your word length, you know it's not appropriate. Instead, say that it's not ready to submit and you're aiming for x. Then don't submit until it's edited, polished, as perfect as you, your critique partners, editors, friends, writing guides can make it.