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.
30-40mmHg is an x-firm or Class II compression. This level of support should only be worn when specifically prescribed by your doctor. A donning aid, such as Compression Assist, Alps Fitting Lotion, or and Easy Slide will make putting these on much easier. Most typically prescribed for moderate to severe swelling, varicose veins, venous insufficiency and to treat or prevent venous stasis ulcers.