Heirloom sizing is a little conflicting amongst everybody. It seems to mostly depend on preference. Some people prefer a little loose. Some like it more fitted.
Here's my .02. I'm 5'10 167 and wear a 38 in ep x Southwick coats. Typically a medium in EPNY and all things epaulet. I got a 40 dark glacier last year but sold it on bc i felt it was a bit too baggy for my liking. So i plan on going with a 38 for a trimmer look