I am a skinny guy and search regular look I'm more worried bout the lengh don't want that it is too small or hanging on the knees
just get a small if you're a pretty skinny guy should work fine. length isn't too different among the two. if you usually rock small button ups go with a small. if medium then go with a medium i guess. fit's pretty true to size.
measurements taken form the site: Small: Chest: 19" 1/2 Sleeve: 26" 1/2 Length: 29" 5/8