Get the best fit by taking your favourite shirt, laying it completely flat, then measuring across the chest starting one inch below the armpit.
From there compare that measurement to this size chart.
XS
Just... no. It's too small, chile. We don't even stock it.
S
Chest: 15" (one side, laid flat)
Length: 27.5" (from shoulder)
M
Chest: 16" (one side, laid flat)
Length: 28" (from shoulder)
L
Chest: 17" (one side, laid flat)
Length: 28.5" (from shoulder)
XL
Chest: 18" (one side, laid flat)
Length: 29" (from shoulder)
2X
Chest: 19" (one side, laid flat)
Length: 29.5" (from shoulder)