I'm a slender 5' 6", 125 lbs, and a small size fits me, with room to spare.  I like it a bit loose -- not tight, tuck-in fitting -- for hiking, traveling and layering over a tank top. Columbia nailed this shirt: it's stylish, designed nicely for outdoor, active wear with excellent upper body protection from solar and bugs (long rollup sleeves, with bottom extending below hips). Great rear vent flap for air circulation, and front pockets. The fabric is substantial (not see-through). When washed, it does wrinkle which a quick ironing can smooth out easily. Color offering is exceptional ! I got the clear blue - vibrant, look-alive appearance. I like styling even when on the trail, and this shirt fits the bill!