Caution: K Swiss Stretch-Fit Visor
I recently bought this visor online with the understanding that "one-size-fits-all" was accurate. After trying it on I found that it was much, much larger than my normal-size head. There's no way to adjust it and polyester doesn't really shrink in the wash.
Basically I advise that you avoid this product unless you have a very large head. (or need to protect your neck from glare).