A Large ski knee pad fits a knee circumference of roughly 40-44 cm, measured around the center of your kneecap with your leg slightly bent. This size is typically best for larger adults. To get an accurate measurement, wrap a soft tape measure around your knee at the kneecap while standing with a slight bend — don't measure straight-legged, since that changes the circumference.
If you're between Medium and Large, think about your priorities. Sizing up to Large gives you more comfort and better circulation for long days on the mountain, while sizing down to Medium gives you a tighter, more secure feel. Just keep in mind that a pad that's too loose can slide off your knee during a fall — exactly when you need protection most. Always double-check the specific brand's size chart, since sizing can vary noticeably between manufacturers.