The following is a four step guide to determine the size of a goaltender's pads.
Step 1 - Goaltender's skate size
Step 2 - Measure from the floor to the middle of the kneecap.
Step 3 - Measure from the middle of the kneecap to desired height on thigh.
Step 4 - Add steps 1, 2, and 3 for estimated size of leg pad.