I just got a stability ball to use on KPC too. The balls I have seen go by sizes of height: The one I got was size S, 55 cm for people 5'0"-5"5" and the size M one was 65 cm and for people 5'6"-6'0", and the size L one was 75 cm and for people over 6'0". So, I would go with a 55 cm ball if I...