I tried on some Ring Jacket stuff at the Armoury in NYC. For reference, I'm a 42 S in most suits and sportcoats; 5'6"; broad in the shoulders, chest and back.
-I was a 42 in the AMJ model sports coat. The length of the jacket is short; I'm 5'6" and it was perfect for me.
-I was an XXL in the "pea coat" (really a bridge coat) and even then over a sweater it's a tad on the snug side, but still comfortable.