First off, let me say that these shirts are incredibly well-made. Having said that, I seem to be a bit of an in-between size. I am a 63 year old male, 5' 8", with a 42" chest and a 32" waist. The large size shirt fits me just about right in the chest but is a little big around the shoulders and, huge in the arms. The waist is slightly loose but acceptable.
The Beyond return policy is fabulous but, I liked the shirt so much that I decided to give it to my 6', 44" chest, 36" waist Son. He loves the shirt, and it fits him well!
I ordered a medium of the same model, thinking it should fit me, since the large is a little too large. The medium fits well in the shoulders, arms and waist, but the chest is a little tight. There's a lot of stretchy material in the chest so it's not uncomfortable, it just looks a bit like I'm wearing a muscle shirt.
I decided to keep the medium, it's for my backpacking trips in the woods so, I'm not really looking to make a fashion statement, I'm looking for durability, performance and comfort.