If you go to the SMB website, or on the back of a bag of SMB boots it should say the measurements for the sizes. I think it depends on the type of boot too.
I have a pretty thick legged Paint that I thought would take a size Large on his back and front, but they were too big so I ended up getting a size medium for both his front and back legs and they fit perfect! (This was with the SMB elites)
Take a ride across the badlands
Feel that freedom on your face - Breathe in all that open space
You'll understand why God made -Those fly over states