This slipper comes in Small, Medium, Large, and Extra Large. If you are on the border of sizes, you could wear both sizes depending on your preference. For example, if you are a size 9, your heel will be at the end of the slipper in the Large. If you choose an Extra Large, you may have about an inch space from your heel and the back of the slipper. In this scenario, the Large is usually preferred. But if you like your slipper roomy, than the Extra Large may be your preference. Remember, that the sheepskin will pack down with time and conform to your foot.