This was listed as 'one size', but I must not be part of that listing, as it was too small, and somewhat uncomfortable for me. I have a 7.5 inch (19.05 centimeter) hat size. This is adjustable, but even at maximum adjustment, it didn't fit me. I CAN say it is windproof, because as I was riding down the street, I could feel the pressure of the wind, but not the cold. It recommends you hand wash it cold, and lay it flat to dry, but I machine washed it warm, and let it hang dry. Since it is made of the same material that wetsuits are made of, (Neoprene) it didn't shrink when I washed it. Just PLEASE make sure you don't have the same large hat size as I do before you order. I wish I could keep this, but it's too small. I guess I'll just find a larger size!