Tennis racket grip size chart:
Grip 0 (4) Junior/very small hands
Grip 1 (4 1/8) Small
Grip 2 (4 1/4) Medium
Grip 3 (4/3/8) Medium Large
Grip 4 (4 1/2) Large
Grip 5 (4 5/8) Very large
If you require anything in between then adding 1 standard overgrip increases grip size by 1/16 and adds 10 grams of weight to the rear/handle side.