hmm - I don't know my head size, I'm afraid, but I use a Giro helmet size M, the Small fit but didn't leave me enough leeway for anything underneath i winter. So I guess I'm a S/M. I wear the regular Buffs, but after a lot of wear they get a bit sloppier. Tehy're too loose for me to wear them around my neck like a neck gaiter, though, they just slip down.