If i take a deep breath and when the tape around my chest is at 100cm, it starts to get a little tight so i went for the medium. I was a little confused at the ridgeline size guide as it said 100cm is a medium so i thought i'd need a large, but i was later told that it would fit a chest size of 100cm. So i went for a medium and there's still plenty of room for a jumper or something underneath it. They're primarily designed for shooting i think, hence the big breast procket so they're supposed to be roomy for lying down and stuff like that.
Jake.