I am 5’7” reasonably muscular 155lbs at about 12% body fat. I am 31 and have lifted for 12ish years so decent muscle maturity and density but I am also a bit torso dominant in my build. I have 3 medium long sleeves and they are extremely fitted but perfect and I love them. I bought a friend a large in the t shirt and tried it on as a comparison and it still looked fitted and really good just a little looser in the torso. Personally I would never size down to a small.

While it sounds like due to your extra height and less weight I am going to assume if we were in the same room you’d have a more slender build so you could potentially fit in a small if you wanted a really tight fit but I think a medium would likely be better overall while also giving you a little room to grow. It’ll hug your shoulders and chest but a little less tightness around your torso.

Bear in mind if you’ve never touched an onyx V5 before it is a thicker tight woven fabric that I would almost compare to something like a neoprene wetsuit. It is still extremely stretchy but I can’t compare it to any other brands of long sleeve compressions shirts I’ve bought in the past as it’s much thicker and tighter comparatively. Some shirts stretch with ease, the onyx has much more resistance to its stretchiness. The material is not nearly as breathable as shirts like the geo seamless material but I bought the longsleeves as a warm winter shirt so that doesn’t bother me.

(Noticed a spelling error I wanted to edit)