2" will be plenty- - - -If you're running duals, it's also a good idea to put a crossover pipe ahead of the mufflers. Otherwise, the side with the heat riser valve runs colder than the other pipe and forms a lot of condensation. That can rust out a muffler and also cause one side to drip water, especially if the vehicle isn't run far enough to get the pipes hot every time it's started.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!