Usually with a stock to mild engine, of roughly 300ci, looking to run duals, 2" will get it done. Don't be afraid to run a single pipe, you'll save money by not buying an additional muffler and save weight, more than likely spend less having the ehaust hung and welded. A single 2.5"-3" is commonly used on an engine of around 300ci.
I have to disagree with John H, all mufflers are not created equal. Whether you choose a bullet style, pancake, louvered, glasspack or chambered, they each have their own exhaust tone and some move air better than others.
What I consider to sound good may sound like crap to you, with that said, good luck.
