A loose one is just fine if you are looking for something easy, but you'll feel a lot better if you find a tighter fit. I'm talking about the right shirt, of course. It's pretty easy to walk into any department store and grab a small, medium or large shirt, but if your body type is more Michael Phelps than Babe Ruth you'll most likely end up swimming in it.
Even worse, tucking in a loose shirt can be a "bold" look. If you choose to disperse the excess fabric evenly around your waist, you end up with the airy muffintop. Alternatively, you can pull the shirt taught around the front for a faux-slim appearance head on, but the excess fabric will bunch into the unmistakable dual fin cadillac look in the back. It's almost like a dress shirt mullet.