Take your choice...some run a single 3" muffler and associated piping....others run two 3" mufflers, while more problably run a pair of 2.5" straight thru mufflers and 2.5" piping ala the Hooker system from Jegs or Summit. Larger you go, the louder it gets. The single 3's sound like an UPS truck to me and they may drone worse than the dual systems. Just depends on what you like and what bugs you, I guess.
You have your choice of stainless for a bunch of money or aluminized mild steel.
ATR makes most, if not, all of the stainless systems sold by vendors, or direct. I have a 2 3/4" mild steel from Cotton on my GN and a 2.5" Hooker system from Jegs on my T.