The one size fits all may not be for everyone but to give an better idea as to how big it is. My hat size is 7.5 and this is slightly bigger than that. I'm happy with how it fits with the face mask up. The sides end about 1.5" below my ears. With the face mask down it fits comfortably without being too loose nor too snug. It is stretchable enough to fit larger heads but will be snug to tight depending on how much bigger your head is.
Compared the the headgear from Seirus, it is slightly larger than their L size, although the Seirus(which I was also looking at but the price on the Carthartt was cheaper) seems to be made of a thicker spandex like material.
The Carhartt tag was slightly off center and the color of the tag didn't complement the color of the headwear I ordered. I just removed the tag by cutting off the stitching(of the tag).