3" on the KA FTW. I've had both and 3" is definitely the way to go NA or Turbo. 2.5" is civic shit.
If you are concerned about cabin resonance. Defenetely get a Resonator.
I would love to see someone use two of these(in 3" inlet of course):
http://cgi.ebay.com/ebaymotors/ws/eB...tem=8000884112
with a resonator of course...