From the website;
The Tilley Hat fits more comfortably than other hats. It’s designed to be worn low on your head and slightly loose. It should be held on by gravity, not by painful pressure on your forehead! For the proper fit, you should be able to easily insert two fingers, flat, between the middle of your forehead and the front of the Hat. The Hat should be loose enough that you can rotate it to the left and right, and lift it up and down, without friction on your forehead. When it’s windy, use the cord!
So regarding size do you buy per the guidance of the website, or being that the hope is always to be in the wind do you get a tad smaller size?