The popular method for finding your tennis racquet grip size is to follow these simple instruction.
Juniors will usually measure less than 4". Most Junior frames are only offered in this size.
If you are between sizes, go with the smaller grip. A slightly small grip can be built up easily with an overwrap. Too large a grip can not be properly adjusted without altering the frames properties. Overgrips can't build a grip effectively more than 1/8" though, because each layer of overwrap adds to the rounding off of the bevel edges on your handle.