I didn't find any size description and took a chance on the XL. I typically wear a 7 1/2+ hat size, this XL is slightly larger and is actually a nice loose fit but not a flopping around fit and it'll never be too tight (should it shrink like many hats do). It can be snugged up with the attached chin strap if windy - I switched it to a tensioner from a chin strap. It will flex some and be larger as I can stick all my fingers from one hand into the hat band while wearing it.

It's a hat that will get dirty and abused, if it lasts for the price then fine and good. The below observations are for your reference.

The brim is pretty wide so it should be a good sun blocker.
Mesh provides very limited breathability.
Could be durable and may last, except stitching looks odd; lots of loose tag ends in stitching that may fall apart; sewing lines are not consistent, uneven and all over the place in spots; logo not centered (should it be?). Made in china.

The hat looks and feels great. Maintains shape, and is comfy.

I really like this hat. The size is little bigger than others.but usually all boony hat is bigfer than baseball hat.
mesh area allow the air cimes in and out..
good sun protection