Tried out the barmitts on the MTB this weekend. Size Large. Mark Junkerman gave me both Medium and Large to try out. THe medium was probably a better fit, as far as ambient space around my hands/bars, but the Large gave a little more room for my thumb while shifting. I rode 3 hours in 30 degree weather and my hands were warm, with just full finger summer gloves underneath. Never been able to say my fingers were warm anywhere below 45 degrees!