There are four sizes for the MERV Masks...

KIDS : 3-10yrs

TEEN : 10-16yrs REGULAR

ADULT : 16yrs to adult (fits approx 90% adults)

LARGE ADULT : larger adults, adults with facial hair

CUSTOM SIZE MASKS can be made for infants, children or adults.
I am a small Australian business owner. 
Information provided is based on personal research, and experience living with a life threatening illness which severely compromises my immune system.
And therefore, should not be considered as MEDICAL ADVICE, purely suggestion.
The MERV mask was initially designed as a product to protect myself, but due to the shortage in availability of masks, and requests from concerned members of our community, I have made it available to purchase.