Got a large  for women's size 81/2 Sorels and they are two big. Got a size medium for a men's 71/2 work boot and they  are  too big.
Also, make sure the boots you are getting them for have some sort of lip on the back of the heel or they will slide right off.
At least if you lose one it's black and you can see it against the snow and ice and go back and track it down.