In general, the larger diameter, the louder the exhaust will be. Longer is quieter, so if you're looking for quiet, make sure it goes the whole way back to the rear bumper. A common way domestic guys make more flow is to cut the exhaust in front of the rear axle. Makes more flow, but also much more noise.
Thicker tubing walls are typically quieter. Mufflers packed with fibrous material (an extreme generalization) flow a little less but tend to make the low-pitch rumble. Another extreme generalization is that you get what you pay for. Glasspacks are dirt cheap, but they flow very poorly and do very little for altering or cancelling sound. Flowmasters are expensive, but do a good job of flowing well and making a distinctive muffled sound... if you like the sound of flowmasters
Start listening to other cars similar to yours. When you hear one you like, ask the owner what kind of exhaust/muffler they are using.