Good question! I thought the same thing and just guessed. My leg is 16.5inches around just above the knee and the large is slightly too big. Hope that helps

you can just measure around your knee, the sizes on the product description are very accurate, so measure your knee and just select the relevant size, I have a 14inch knee, so I bought the medium and it fits a treat.

This is a heavy brace and I've only ran up to 12 miles in it so far in training runs. It was comfortable and I've no doubt it will remain so for longer runs. I'm in doubt however as to wheather I should wear it for my first marathon which I'm training for. If you are looking to set personal bests or run at speed I'd probably consider a lighter alternative for events.