Grab a measure tape.
(or if you don’t have one handy, grab a string and a ruler)
Measure around your head ( in inches ) taking the measuring
tape just above the ears and about an inch above the brow line.
Select your head shape
On a scale of 1 - 5 - how much hair do you have?
Your size is:
Your helmet should fit snug on your head, so that you can wave your head to either side without it shifting.