The Key factor is the overall length. The length can be divided into three sections:
B.Instep to knee
C.Knee to top or thigh rise
The proper length of the goal pad boot is determined by the goalie's skate size. The next key area is the instep to knee that determines the shin length. Finally, the knee to top or thigh rise is a personal measurement depending on how much pant and pad overlap the goaltender prefers. A general rule is 7" to 8".
A.Skates size: provide the skate size for proper boot fit.
B.Instep to knee: measure from instep to the front center of kneecap while leg is in slightly bent position.
C. Thigh rise: measure from front center of kneecap up the thigh to desire height.
A + B + C = Estimated size of pad
If fitted properly, the goalie's knee should rest directly behind the center knee roll with the pad securely attached to the goal skate and properly strapped on the goaltender's leg. Generally, this will ensure that the pad breaks (bends) at the knee/shin and ankle as designed for optimal performance.