McDavid website states sizing is:
Size Calf Measurement
S 10" - 12"
M 12" - 14"
L 14" - 15"
I have a 14.75" calf, I ordered a large, McDavid's website even says to order a size larger because they run small. Yet when they arrived they felt looser than socks especially as they taper lower. My old wetsuit gives my legs better compression... if only I could run in that! The quality is also not what I expected for something that costs $30. I could be wrong, but these don't seem like they will make it more than a couple months of near daily use.
I am going to go with the McDavid Calf Support (441) instead. They are inexpensive, made of neoprene and will provide substantial support. If you actually have an injury I would suggest the same.