Size Small ski knee pads are designed for a knee circumference of roughly 33–36 cm, which typically fits smaller adults and some teens. To find your measurement, wrap a soft tape measure around the center of your kneecap with your leg slightly bent—this mimics the flex you'll have while skiing and gives you the most accurate reading.
If you land right in the 33–36 cm range, Small is likely your best bet. But if you're between Small and Medium, consider how you want the pad to feel: sizing down to Small gives a more secure, locked-in fit that won't migrate during a fall, while sizing up to Medium offers a bit more comfort for all-day wear. Just keep in mind that a pad that shifts out of place defeats its purpose.
One important caveat—sizing varies between brands, so always check the specific brand's size chart before buying. A Small in one brand might fit differently than a Small in another.