diameter wise, you'd be looking at 2" - 2 1/4". i've got a 2" cat back on mine witha resonator and a standardish muffler with a small tip and i'm quite happy with the sound of it as it isn't terribly loud.
if you don't want loud then steer clear of cannon's and just get a plain muffler with a tip.
i know lucid has extractors on his corolla gti and he said they made a difference over the standard manifold.