To obtain proper fitting of the shot to the ability of your athlete. 1. Finger length is critical. The shot must rest on the Pad (callus) of the palm and the fingers must be up the side of the shot. Throwers with "shorter "fingers may find that the smaller diameter shot feels better at delivery & those with longer fingers may like the feel of the larger diameter. 2. The final decision is a personal choice by the thrower and you may need 2-3 diameters to fit the whole team. From a Biomechanical point of view, the larger shots give a greater contact area & push zone.