resonator....... you mean the catalytic converter. That thing that you bolted your cat back up to? It looked like a honey comb thing if it was still intact ( probably was not if it is cracked right now).
w\e its personal preference. the smaller diameter you have the quieter it will be, although it will create a change in pipe diameter disrupt in exhaust flow. Its not really important, the exhaust system you have on there is not tuned from the header back. Id guess go with a 2.5 -> 3 in resonator so everything bolts up.
or are you talking about the resonator \ muffler on the catback its self?
No the the cat but when I put my exhaust on it was hollow so I asumed it was a resonator. But what about back pressure since I lost some with the exhaust?