I bought this to go with the outfit I'm getting ready.
I'm 5'4", about 165 lbs, and 34 - I have a large and it fits perfectly. It's nice and comfortable, but has a lot of elasticity; I doubt it will fit well even with weight fluctuations.
Honestly, I'm amazed how comfortable it is. This is probably the softest shirt I have ever owned.
So far it has held up well in the wash.