The popular method for finding your tennis racquet grip size is to follow these simple instruction.
On your playing hand, your palm has three main creases. Hold your hand flat, with the fingers alongside one another.
Measure from the middle crease of your palm, up the line between your middle and ring fingers, to a point equal to the height of the tip of your ring finger. Typically women will measure between 4 1/8″ and 4 3/8″, men between 4 3/8″ and 4 1/2″.
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.