A while back, I was filling out an application, and someone asked how long the essay portion should be. The answer: "Writing should be like a miniskirt: Long enough to cover everything; short enough to be interesting." I think that was the best advice ever. In fact, it earned some coveted space on my board of awesome things.
A lot of agents mention on their blog what would be about the right length for a book, depending on genre and intended audience. (A hint, anything over 100k is not a novel, it's a paperweight.) I have heard agents say that after certain word counts, they just put down the query, because they know the book doesn't have a teardrop's chance on a hot stove.
There's some disagreement on the particulars, but basically any agent will tell you that the key is not to waste any of your words. If you MG novel is 30k, it might look short, but maybe you've written the perfect 30k MG novel. If you're writing an 93k YA novel, then those 93k better be worth getting through. How do you think Stephanie Meyer gets away with 105k books? Because no teenage girl was complaining about any of them.
Every word should be covering something that needs to be covered. It should not be there covering things that are best left uncovered. This would be one of those areas where underwriting comes in handy. Saying less to mean more (something that I can admit I don't do perfectly. Yet.) is a great way to tighten up your prose and create subtle works.
My first draft of The Thief Book, I estimated, would probably be around 50k (I was so wrong about that guess), and I figured it would end up closer to 60k once I did all my adding and subtracting and general changes-making. By the time I'm done, 60k is about where it should be. In my mind, that's a reasonable length for a YA book. Other agents might have their own thoughts, but I figured that that was about the right length to cover what I planned to say.
How do you judge word length? How long to your books average?