For sizing please measure around your face from masseter to masseter (roughly the point of the jaw around an inch below the ear)
S/M = 8.5" - L/XL = 10"
As a general indicator the female staff at Infectious Clothing wear this mask in size S/M, while the male staff (particularly those with facial hair) find the L/XL to be a better fit. However if you are unsure which would fit better for you we definitely recommend referring to the measurements above