As thin guy without a lot of hair, I usually by a S/M and did so with this hat. The diameter is good, but the brim appears to have been sewn on from a larger hat, giving a "too big" look to the hat overall. The hat is cool, blocks the sun / rain well, and overall build quality it good. I wear this for commuting but wouldn't wear it around on weekends as the appearance is a little strange.
This review was written in the old system and had content requirements that are different than reviews written today.