Before selecting a size, we would advise to measure either the users inside leg length, or arm length, and compare it to the ball diameter. This should give you a good indication of how the user will interact with the ball.
Sizes (dia x length) :
Petite – 30 x 50cms
Small – 40 x 65 cms
Medium – 55 x 90cms
Large – 70 x 115cms
X large – 85 x 130cms