I ordered a S/M and wear a New Era fitted 7 1/4 baseball hat. It fits well - not excessively loose and not so tight that it leaves marks on my forehead.
My concern echoes the below comment - and I keep wondering if wearing it on a breezy day might cause me to run down a hill after it, or just sacrifice it to the mountain gods.
It's a great little bucket hat - and I'll risk it on hikes because it does the job of keeping sun out of my eyes. However, it won't replace my standard Nike Detroit Tigers hat that I normally hike in just yet...