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