As a tall thin person, I've always worn "Large" sized shirts (tshirts, button ups, jackets, etc). Over the last two years I noticed that the button up long sleeve shirts that I like have almost exactly the same overall length, sleeve length, and collar. The only obvious difference is that they're much larger around in the stomach area.

About a year ago I picked up a size L insulated flannel shirt and thought "sleeves/length is perfect, this'll do" and the person with me told me I had a carnival tent in the back that I probably wouldn't have realized until later. So I picked up the M size, sleeves/length were the same, but still a lot of extra fabric around midsection. Picked up a S size... perfect.

That blew my mind a little bit that the overall length and sleeve length of the Small to Large were almost identical, they just tailored in a lot more fabric to account for big bellies. Makes me think that probably sucks for people who are not tall and wear a Small....