At first I was a bit sceptical when buying this as I didn't know whether to go for Medium or Large. I tend to be in the middle of the two so in the end I went for Medium.
so Bought a small and a medium to try out for my husband. He kept the medium and I kept the small.
For reference, he is about 6ft and 150ish pounds. He is a lean, athletic build.
When I let him tried this on him thought it was too small... but, it's obviously supposed to be skin tight so he gave it a go when cycling outside.
he said he didn't even notice I had it on, I was worried it would be too tight but the fit is perfect. It keeps the instant breeze off your arms and also keeps me nice a cool in the sun. A great addition to my soccer match, definitely recommend.
He typically wears a medium sized shirt, and the medium fit as expected- snug, but not overly so.

Got these to go under his work t-shirts to protect him from the sun, as he will not wear sunscreen.
Works great! He said when the wind blows, they are great at feeling cool. However, if he is indoors or there is no breeze, borussia dortmund black jersey they are "hot as h-e-double hockey sticks" in his words. They have definitely helped in that he no longer comes home looking burnt if he is installing windows or glass outdoors.

I kept the small for myself for when I mow or do other yard work, and it fits fine. For reference on how it fit me for women looking at these for myself- I am 5'1" and around 120ish, size c cup, and I typically wear medium in women's shirts, small in men's. Fits good, not too much compression in the forearm region on me, but this was made for men, and I like the fit.