This shirt runs a bit small. I'm not a super skinny dude but a medium usually fits me with a little bit of room to spare. I'm 5'9" and about 175 lbs. I've got an average build to athletic build but a little extra around the waist. I can wear this shirt but it's tight, especially around the chest and arms. I'd call this a slim fit Medium and I think a Large would have fit perfectly.