I love this shirt! It's so soft, moves well, I love the razor back and the fact that it's long! I have a very hard time finding tanks that fit my torso and this is perfect! I will be buying more!
For reference I'm just over 5'8", 158lbs, athletic build, size 8-10 pants and typically wear a 34D bra on the middle or mid/end of the strap. If you have a larger chest and are closer to a 35 or 36 I would definitely get the large, it's not that much bigger, it doesn't sit funny or have loose areas, for me it's still form fitting and holds me in extremely well.