Size S but really oversized so please refer to all the measurements:

Armpit to Armpit : 22"

Length in back middle without the collar : 28"

For Reference : My son is wearing a size XS in the picture but would usually be a size Medium. As you can see it is still loose.

Limited-edition collaboration with a classic sporty vibe
Lived-in, vintage wash
Ultra-soft jersey 
Snoopy playing soccer
Football jersey styling and stripe detailing.