Earlier this month, I offered my limited knowledge of picture book writing. This weekend I can direct you to a couple of blog posts that will offer you more. They're both at Writers' Rumpus, and they're both written by Joyce Audy Zarins.
Why Thirty-two Pages? deals with the length of picture books, at least in terms of pages, not word count.
3 Ways to Pace Your Picture Book is a very meaty piece of writing on pace, how drawn out or rapid writing feels when being read.