Just Big Enough, by Mercer Mayer, Harper Collins, 2004.
I liked this book -- I like all of Mercer Mayer's books -- but this one wasn't a favorite. The topic was good, and Little Critters attempts to grow were funny, but I wasn't satisfied with the resolution. In real life, the big kids would probably win a running race. Now if the kids had raced through tunnels or other small places, that would have had a different outcome and Little Critter would have been happy that he was little, or just big enough.
The illustrations, however, are perfect.