Petite or short length compression stockings are sized to fit a shorter than average length 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.
20-30mmHg is a firm or Class I compression. This compression level is prescribed to treat an extremely wide range of venous and lymphatic issues. Some of the more common are, mild to moderate edema (swelling), varicose veins, spider veins, post-sclerotherapy and for mild to moderate venous insufficiency.