Junior Racquet Length Guide
If they're big for their age then go up to the appropriate age category. For example, if a six year old is the size of an average eight year old then go with the racquet recommended for an eight year old. If you're still unable to decide between two sizes err on the side of the larger racquet as the racquets are quite light and easy to play with.
Age: Racquet Length
0-4: As small as you can find.
11-12: 27" (Adult Size), less than 135g, head light balance