I swear by these Under Armour tops for training. I have 5 of these now and have just moved down to a medium from large. The material used is great, it's elasticated and light, non-chafing, keeps you cool and has no tags. The fit is not compressive. It's very unobtrusive when you're on the weights or hitting cardio and doesn't get heavy with sweat. The drying time is extremely quick.
As far as measurements go, I think they run a little large. To assist you in sizing, for reference I am 6' and the large was a bit snug back when I was 16 stone (225lbs) and (slightly ;)) round around the middle, now I am in better shape and weighing 13st 4lbs (186lbs) and the medium size is a great fit, snug (not tight) around chest and shoulders and with room to breath around the waist.
Highly recommended, remember to wear something warm over it when travelling to the gym though or you will freeze.