There is a size guide in the PDF file below the product description. You have to measure around the ankle and heel.
Size 1 fits 10 1/2 - 12 1/4 inches
Size 2 fits 12 1/4 - 13 1/8 inches
Size 3 fits 13 1/8 - 15 3/4 inches
Please note that you should consult a trained professional for fitting and customization. BraceLab cannot accept brace returns that have been worn or altered.