I would say that I have a medium to large cranium. Baseball cap size is 7&1/4 - 7&3/8. The new Radars fit me perfectly. From some others who've reviewed them, I thought they were going to be too tight, but I tried them on at a bike shop and the fit was amazing. Snug, yet flexible, like they're giving your head a gentle hug, but not the vice-like grip of say a SJ 1.0. Know what I mean? So glad I picked them up! Unfortunately I got them in Metallic Red/+Red so I feel obligated to keep them in the box/display case and never wear them (I also want the Retina Burn, but they have the same issues). Some day I'll have to pick up an old beater pair to wear when I go biking (like white chrome/blue irid).