for most people with either the 2 litre or 2.4, a press bent 2.25" will do them fine. it will give great torque & peak power increases, for the crazy/stubborn buggers 2.5 or 3". although none of us here have 3". yet.
exhausts are custom made at the shop, only thing you have to decide on is the size, muffler & tip. muffers range from bugger all to quite expensive. you'd be looking at around $300 for a 2.25" exhaust with a good muffler and shiny chrome tip.