I bought a Bell sweep R helmet (size small) on ebay. The size specified was 20-21.75" around the head. I measured my head and was at 21.25", so I figured I was good with a small. I try it on and it is tiny, almost like it would fit a 5 year old better. I will exchange it, but I don't know if I should go with a medium (21.75-23.25") or large (23.25-24.75").