Petite or short length compression stockings are sized to fit a short leg. For knee highs, measure from the floor to the bend of the knee. If 15" or less, you will need a petite length. For thigh highs and pantyhose, measure from the floor to the crease of the buttocks. If 28" or less, you'll wear petite.
15-20mmHg is considered a moderate level of compression. This is the compression your doctor would typically recommend for travel, standing or sitting for long periods of time, mild swelling and varicose veins, and as a way to improve the circulation in your legs and general leg health.