To check you have the correct grip size: you should be just about able to place the index finger of your non-racket hand inbetween your fingers and palm when hold the racket normally in your racket hand.
Age 10 + / Height 140cm-150cm - Choose a 26" racket
Age 8 -10 / Height 130cm - 140cm - Choose a 25" racket
Age 6 - 8 / Height 120cm - 130cm - Choose a 23" racket
Age 4 - 6 / Height 105cm-120cm - Choose a 21" racket
Age 3 - 4 / Height 90cm - 105cm - Choose a 19" racket
Age 3 & Under / Height Up to 90cm - Choose a 17" racket