Measure the chest by taking your favourite shirt, laying it completely flat, then measuring across the chest starting one inch below the armpit.
XS
Chest: 20.75" (one side, laid flat)
Length: 17.25" (from shoulder)
S
Chest: 21.75" (one side, laid flat)
Length: 18.25" (from shoulder)
M
Chest: 22.75" (one side, laid flat)
Length: 19.25" (from shoulder)
L
Chest: 23.75" (one side, laid flat)
Length: 20.25" (from shoulder)
XL
Chest: 24.75" (one side, laid flat)
Length: 21.25" (from shoulder)
2X
Chest: 25.75" (one side, laid flat)
Length: 22.25" (from shoulder)