The XS/S size is designed for torsos measuring 36–43 cm from the base of your neck (that bony C7 vertebra) down to the top of your hip bones. This size typically fits youth, smaller adults, and many women. Don't just go by your overall height—torso length is what really determines pack fit.
If your pack's back panel is too long, the hip belt rides above your hips instead of sitting on them. That means your shoulders end up carrying the full weight of your shovel, probe, layers, and water—all day on the mountain. A properly fitting XS/S pack transfers that load to your hips, which makes a huge difference on long tours or resort laps.
To measure, have a friend run a tape from the prominent bone at the back of your neck down to your iliac crest (the top of your hip bones). If you're under 43 cm, go with XS/S. Also consider women-specific models, which pair the shorter torso length with shoulder straps and hip belts shaped for different anatomy.