The balmacaan this year is absolutely true to size. I don't think we've had one return on the grounds of sizing.
So, if you go to size L, you will have a coat ... one size too big. It's a raglan-sleeve overcoat, though, so isn't as size-specific as e.g. a tailored jacket. I happen to think they look quite good "oversized", but I'd really only consider it if you're prone to wearing lots of layer beneath.
Cheers both.
Living in London I'd wear a jumper underneath, but not much more, doesn't get all that cold here comparatively speaking. I reckon it's best I wait out for a future version in an M.