I suggest that you use 2 and a half inch piping. I've heard a lot about the backpressure stuff also, and from what I hear 2 and a half is the best size because it provides enough backpressure to keep the pressure inside the engine at optimum levels but enough opening to leave the engine clear of too much air pressure. My friend has an 03 Civic Ex and He got custom exhaust with 2 and a half inch piping with an AeroSpeed muffler. The guy he got it from told us what I just said at the beginning. Maybe it's, bigger is better
but I just heard 2 and a half was the best.