To get the right fit, you have to measure just as they show in the product pictures. Use a measuring tape (fabric, and not metal), put it under the corner of your heel and up around to the front of your ankle.
I bought a size medium with my measurement of 11" and it is sufficiently tight and compressive. Even without the straps.
I also measured around my ankle, which was 8.5", If you have a puffy, swollen, or wide fore-foot, but your ankle still measures what is indicated in the product pictures I would go up a size.