Size L headguard should fit most average-height people.
If you want to be sure, you can measure your head circumference (above your eyebrows and ears; if possible, pull your hair up to get a precise measurement) and use the table below to pick the right size.
19.0”-20.5” Circumference | Small
21.0”-22.0” Circumference | Medium
22.5”- 23.0” Circumference | Large
23.5”-24.0” Circumference | Extra Large