Go to any bike accessories shop & test fit your head & have them help you determine your head size
& either support your local shop or order online once you know what size your head is & make sure it the same brand/mfg cause sometimes brand/mfg sizes varies.
Me personally, I would get Carbon fiber (strong & light weight) &/or modular one (convenience) if I can afford it.
Why I got the one I got? because I was cheap & I was on a budget.
