A medium ski knee pad is designed for a knee circumference of roughly 36–40 cm, which fits most average-sized adults. To get your measurement, wrap a soft tape measure around the center of your kneecap with your leg slightly bent—this mimics the flexed position you'll actually be skiing in.
If you fall between sizes, think about what matters more to you. Sizing down gives a tighter, more secure fit that won't shift during a fall, while sizing up adds comfort for long days on the mountain. Just remember that a loose pad can slide off your knee when you need it most.
Always check the brand's specific size chart before buying. Sizing varies a lot between manufacturers, so a medium in one brand might fit differently than a medium in another.