That's another thing, sizing. M is usually for medium, but MRL's medium is deceiving because the chest circumference would suggest a Large rating, but I accidentally bought the L/XL one because I thought THAT should fit. It was like trying to put on a fully-deployed parachute as a dress. I'm being facetious, just know that I was literally swimming in it. But I'm not that big, at 6' and weighing in at 175lbs...so I learned. Always go with the M, especially for you Conall, as I know you to be slimmer than myself. OR...you could have one custom-made for you, just need a seamstress of some skill and a fairly concrete idea of what it is you're wanting.
I still haven't asked my Mom to do one up for me yet...I'm waiting to see what we decide to do with the Order, as I may want a surcoat made especially for it.