Heather Castle's blog has some great information for aspiring children's book illustrators. Her latest post tackles the question, "What size should I illustrate for children's books?"
The biggest takeaway from this is don't provide finished illustrations for the whole book. It will take you forever and won't help you sell the book. Provide sketches and perhaps one, finished illustration.
You never what to hear from and editor who says, "The story was great, but the illustrations are a bit off." I've seen it before. It happens. Your illustrations may be great, but not quite the angle the editor would like to go. Or it could go the other way. The publisher loves your illustration style but thinks the story is drivel. If that's the case, you have quite a portfolio piece, but a couple of illustrations would have sufficed.
It's the KISS method, friends! Keep it simple, Silly!