Here's my newbie take on the FX-39. My head measures 22.5" which is exactly in-between the Small and Medium. The Small felt pretty snug (a good snug) in the store and the medium maybe be a tad too big. I decided to go with the Small AFX in Hi Viz yellow. I got it home (ordered from DennisKirk.com actually) and it seemed a little too tight but I think it will break-in to about perfect. Actually, a day later of just wearing it around the house and it's feeling better already. I think the store version was broken-in a bit from all the test fitting.
I also tried the Fly Racing Trekker in the store as my 22.5" head is exactly in the middle of their Medium range. The AFX Medium did feel a tad bigger than the Fly Medium which is consitent with the info found online. But I felt the Fly and AFX Smalls felt about the same.
I like Fly's version of Hi-Viz yellow better than AFX's, but I'm really digging the AFX ratchet strap, so the strap design won out over the color for me. I still plan to add some Hi-Viz Scotchlite stickers to the helmet anyway.
Just need to buy a motorcycle now....