One reviewer suggested that medium size would fit someone who is 5'6" and 165 lbs and indeed my cotton undershirts are size medium. Well not all bodies with the same height and weight have the same shape and the medium was too small (more like a slim fit). The 8% Lycra caused the undershirt to fit too snug for comfort. I ordered the large size and hope for a better fit. The fabric will be great for travel.