Every once in a while people ask me for advice on which size they should pick when getting one of our tshirts. They are sometimes stuck between picking a size they used to pull off and the size that better reflects their current body. I always ask them pick the size that they're most comfortable in.
The reasoning is simple. If you feel comfortable, you are going to feel more confident. And when you feel more confident, you are going to appear good looking. It always works. :)