Size S hip protectors are designed for small adults with a waist of about 70–80 cm and hips around 90–97 cm. If your measurements fall in that range, a small should position the protective pads right where you need them — over the greater trochanter, which is the bony prominence on the side of your upper thigh. That alignment is everything; if the pads don't sit over that spot, they can't do their job.
The biggest mistake smaller skiers make is guessing their size based on regular clothing or choosing a size up for comfort. If the protector is too loose, the pads can shift off target during a fall. Too tight, and they may ride up above the hip joint entirely. Measure your waist and hips at their widest points, then double-check against the specific brand's chart — sizing varies a lot between manufacturers.
When in doubt, go slightly snug rather than loose. Compression-style shorts should feel firm but not restrictive, keeping the pads locked in place run after run.