I'm 6'1" 185lbs and the medium is a perfect top layer over a t-shirt for practice or working out. I like it so much I wear it casually too. If I wanted to wear this over a jersey I'd likely size up to a large. I like that it has a small split hem on the side and that it's slightly longer in the back. Sure that's a small feature but helps give just a tad more room for mobility and provides full length coverage on the back without getting in the way in the front. While I haven't tried this yet, I think it's slim enough that you could wear it under a jersey in a pinch on a colder day ~40 degrees (the reality of playing baseball in the spring and fall in the mid-atlantic and northeast) if you forgot or didn't have some type of Nike or Under Armour type cold gear base layer.
