These are made to be worn over all your clothing as a waterproof layer. The best measurement is taken with all your gear on, then measure around your middle at the thickest part, then pick the next size up on “chest” measurement.
Back: From collar to hem along the back.
Chest: Around the chest just under the armpit, or the widest part of your middle.
Sleeve: From the shoulder seam (which is about 10″ out from the collar)to the cuff.
Medium > Back: 37″, Chest: 58″, Sleeve: 18″
Large > Back: 40″, Chest: 70″, Sleeve: 22.5″
X Large > Back: 45″, Chest: 74″, Sleeve: 22.5″
XX Large > Back: 46″, Chest: 82″, Sleeve: 22.5″
XXX Large > Back: 46″, Chest: 89″, Sleeve: 29″