I ordered three sizes, S, M, and L, to be safe. I normally wear a size medium T-shirt, and a small or medium dress shirt. I'm tall and thin, so most long sleeve items have sleeves that are too short, so I'm used to that. I'm about 6' tall, 145 lbs. (very thin), normally wear a 36 long suit, 30" waist, 17.5" shoulders, 15" neckline. Here's what I found:

Small was way too small, didn't even try to put it on.

Medium was close to fitting. The shoulders were just a little too small, but the waist was about right. Sleeves were about 1.5" too short for me, but standard length for shirts of that size.

Large fit best. Shoulders were good, but sleeves were too wide, waist was too big. Sleeves were just a tad too short but otherwise fine. I was able to do a quick and easy tailoring job, narrowing the sleeves, chest, and waist, and now it practically fits like a glove.

The pin clasps on the badge are a tad loose, not especially high quality, and one of them won't stay on. I was able to replace it with the back of a different pin, so I took off one star for that. Looks great otherwise!