EA, about that 6,000-word picture book you mentioned a while back. I know most picture books are under 1000 words these days, but I've noticed that picture book biographies are often longer and aimed at older readers. How many words max would you want to see in a picture book bio?
This is a separate category. The 6,000-word picture book was hilarious because the topic clearly marked it for pre-K.
I love that more people are offering picture books to older kids. Most of those people are still teachers, though, especially in topics that have been fictionalized. Straight nonfiction picture books have had a pretty good spread into late elementary for a while.
You want to be careful about biographies, though--only make it a picture book biography if you really want it to reach into early elementary. Once you hit fourth grade, most teachers are demanding that biographies read for school have at least 100 pages. That cuts the picture books out of the equation.
The trick is not to say 'this category means this word count', but to picture the book's use. Longer nonfiction picture books will be read by the child, or browsed by parent and child. You still can't have an enormous word count, but you can have a darned high one if it's broken into appetizer-sized chunks. Think of the popularity of the Eyewitness books. Those have big word counts, but they make it easy to read a little here and there, and there's still a great deal of space for pictures.
But if the book is best used as a read-aloud--and that applies to picture books for the 0-7 set and to picture books on topics best aimed at middle-schoolers (because seriously, no middle-schooler is going to pick up a picture book him/herself), then you have to keep the word count down to something you can read aloud in 10 minutes. And 5 minutes is much better.
The real killer in any kind of picture book is big blocks of text. Don't do this. Either have a nimble little text, or break your long text into nimble little pieces.