Approximate measurements - width armpit to armpit when laid flat:

 

Fleece:

 

S = 22 inches

M = 22.5 inches

L = 23 inches

XL = 25 inches

 

Polo shirt:

 

S = 20.5 inches

M = 21 inches

L = 23 inches

XL = 24 inches

XXL = 24.5 inches