Our son has a used GMax size Medium that fits me, but a tiny bit tight. I ordered the Large based on reviews saying this helmet ran small. Nope, had to return it for an X-Large which fits perfectly. When I talked with a GearHead with MotoSport, I was thinking of going to an XXLarge, but with his assistance, we went with the X-Large and it was perfect. The MotoSport GearHead's are so helpful!