Well there isn't any real measuring necessary really with a helmet for road ridding other than when it is on your head it should feel "right"....and that is up to YOU! Do you not know what feels good on your head or not? When the helmet is on your head and your head is forward on the bike in a natural ridding position then you should be able to see under any helmet as if you are looking over the top of a pair of glasses and the helmet should not impede this line of sight. The straps should fit around the ears and not over them and it should have little movement front to back and side to side (shouldn't be over tight on your head) you don't want it to squeeze your head to the point of a headache. Get the Giro that feels best to you at a price you can handle, its really that simple.
