I finally got my cousin's wife to try on my 5.11 Holster Shirt. I wear a Medium and, while she wasn't swimming in it, it was definitely too large for her. 5.11 makes a Small size. According to 5.11's site:
It's best to find a shop that carries this and to try it on. It's a compression style shirt so it should fit like a second skin. Plus, when I wear it in San Francisco (I'm carrying my iPod in the pockets, not a gun), I'm a lot warmer without the extra bulk of a sweatshirt. If you do get one, it's best worn with a button up shirt/blouse in order to access the pocket. A pullover defeats it's purpose of a quicker draw.