I'm a size M guy, everything from head to toe is M. If M doesn't fit, I will try S, but most of the time that is too small, so I just skip that fit or brand. Sometimes the size difference is so ridiculous it might as well be two different shirts. One time I tried a polo in M and it looked like an oversized 90s hip‑hop shirt on me so I tried the S and it was so tight it looked like swimwear lol.