I bought the L/XL hat which fits 56-62cm heads. My head measures 58cm and have worn 59cm helmets for years.
The L/XL hat was way too big and has no adjustment other than tightening the chin strap. The return process has been reasonably painless so far. I'm not sure I'll order the SM/M (52-57cm) hat but customer support says it should fit.
In the meantime, I pulled the certified shell out of a bump cap I use when doing work in places where I can hit my head (which has happened often enough that I bought a bump cap :-( ).
The protector, or the cap can be bought for $20-30
prochoicesafetygear.com/BCABSPI put it inside an old Rip Curl surf hat I have and it fits perfectly. I think my hot weather riding protection problem is now solved.
There's only one remaining problem. Bump caps protect the top and front of your head. There's a huge cap down the back of the protector. The majority of bumps to the head foiling have been gentle taps to the back of my head when the board sneaks up behind me. I experimented ages ago with DIY head protection and made various shapes out of closed cell foam that fit inside caps. It would be easy enough to make an insert for the back of the sun hat.