The size of the helmet is of paramount importance and, of course, it must fit the user's head perfectly, neither tight nor loose. If it is loose, the helmet can slip off in a fall or get stuck on one side. If it's too tight, it creates constant pressure on your head, which is unhealthy, if not uncomfortable, in the long run. While trying on a helmet, wear it and move your head up and down and left to right. The helmet must remain in place and not move during these movements.