A size M hip protector is designed for waists around 80–90 cm and hips around 97–104 cm, which covers many average-build adults. The key is making sure the protective pads actually sit over the bony prominence on your upper thigh—called the greater trochanter—because that's the area most vulnerable in a fall.
To check your fit, measure your waist and hips at their widest points with a soft tape measure, then compare against the brand's specific size chart. Sizing can vary quite a bit between manufacturers, so don't assume your usual clothing size will match. If you're between sizes, it's generally better to size up slightly—the shorts should feel snug but not restrictive, and the pads must stay put when you move.
A properly fitting M will keep the pads aligned during active skiing without riding up or shifting off target. If the pads drift above your hip joint, they won't protect you when you need them.