Under the description it says:
Easy sizing: S(6 3/8-6 3/4), M(6 7/8-7 1/8), L(7 1/4-7 1/2), XL(7 5/8)
Use the measuring guideline on the site and then pick the size that that number falls under. For example, if your measurement gives you a hat size of 7 1/8, then get a Medium.
Click the icon that says Hat Size in the section where you choose color and size for the measuring guideline.
Hope this helps.