To Size for this Helmet:
You want to measure around the head / face, starting underneith the
chin, up one side of the head just through one ear, over the top of the
head, and down the other side just through the other ear, and then
ending at the chin to make a complete circle.
Use a soft ruler, or a piece of string or yarn to measure afterwards,
and make sure you keep whatever you're using to measure snug on your
head/face, because if it's loose at all you'll wind up with a mask
that's too large!
Also note: Your helmets are slightly adjustable! If you receive your
helmet and are unsure if it fits, try to adjusting it before you get a
larger or smaller size! Squeeze or stretch it to bend it to make minor adjustments.