I could be wrong but I am pretty sure they only make a Medium and Large? At least that is was Kurt has on his site as well as Blackovis.com
I wear a 12 boot. Use the large gaiter. IMO, best gaiter out there. They have an extra Velcro strip that allows you to custom fit the gaiter around your calves. They don it slip down like others or cut your circulation off because your tighten the top straps are tight to prevent from slipping.
Have used the gaiters for two full seasons now and they’re still going strong.
SIZING
Medium - Fits Boot Size 10 and Smaller
Large - Fits Boot Size 10.5 and Larger