2.5" according to most of what i've read is good for about 450hp or so worth of air.

Another things to consider are your transitions for your tubing... you should always have as gradual a transition if you can (within reason) when changing tubing size... or going from your compressor outlet to the proper inlet tract tubing size.

04 Subaru WRX "Eurosport bling bling fast and furious tokyo drift" "They have diarrhea of the mouth, and constipation of thought"