Measure your face as shown in the above diagram
From nose bridge to chin tip.
Or
From center of nose to 1" under chin.
Face measurement range:
XS = 3 1/2" - 4"
S = 4" - 4 3/4"
M = 5" - 5 1/2"
L = 5 1/2" - 6"
Mask measurement is approximately:
XS = 4 1/2"
S = 5"
M = 5 1/2"
L = 6"
Additional tips, if you are a:
Petite adult female - go with size S.
Average adult male - go with size M
Larger woman - go with size L
Most Men - Size L