Measure the thickness of the ear foam to determine your size if the label has worn away.
Small - 1/2", Medium - 1/2", Large - 3/8", XLarge - 9/16".

Stock Foam Thicknesses: 

Small:
Forehead: 1/2"
Top of Head: 1/2" - May require small cut to fit, instructions will be included with your kit.
Back of Head: 1/4"
Ears: 1/2"
Cheeks: 1/4"
Chin: 1/4"
Backplate: 1/2"

Medium: 
Forehead: 1/2"
Top of Head: 1/2"
Back of Head: 1/4"
Ears: 1/2"
Cheeks: 1/4"
Chin: 1/4"
Backplate: 1/2"

Large: Forehead: 3/8"
Top of Head: 3/8"
Back of Head: 1/4"
Ears: 3/8"
Cheeks: 1/4"
Chin: 1/4"
Backplate: 1/2"

XLarge: Forehead: 9/16"
Top of Head: 9/16"
Back of Head: 1/4"
Ears: 9/16"
Cheeks: 1/4"
Chin: 1/4"
​​​​​​​Backplate: 1/2"