You will require three measurements.
1. Your skate size.
2. Measure from inside ankle bone up to the center of the knee cap with the leg bent slightly forward.
3. Measure from the center of the kneecap up to the desired height on your thigh. This will typically run between 6 and 9 inches.
Add these three values together for your recommended goal pad size. Ideally, the player's knee will fit into the middle knee roll.