Approximate size guide:
S - 6 3/4 to 6 7/8
M - 7 to 7 1/8
L - 7 1/4 to 7 3/8
XL - 7 1/2 to 7 5/8
XXL - 7 3/4
The closer you are to the upper limit the better the fit.
Foam tape is sent with each hat to snug it up if necessary.
The venting on each hat is the same but the exact placement/distance from the brim varies.