If you only plan to do a mild build on the car, with bolt ons, go for a 60mm exhaust. You wont need anything bigger than that. If you plan on doing a serious all motor build, I recommend a 2.5" exhaust. And if you plan on turbo, go 3". From there, decide on a muffler style. Straight through will provide the most power, but (because it is basically a glasspack), it will be the loudest. The next is chambered. The Chambered exhaust uses chambers to muffle noise, but it does cost some power. And lastly is the twin loop. The twin loop is one of the most quiet exhausts, and some companies claim no loss in horsepower with one. A twin loop muffler sends the same exhaust gasses through the muffler twice, before releasing it.
I have a serious all motor build, so Im going with a 2.5" stainless steel piping kit, a 16" resonator, and a stainless steel twin loop muffler (SNL).