Until you are certain that you'll eventually be boosted, don't buy anything w/ a diameter larger than 2.5". Of course, for our cars, this isn't an easy thing to do since most companies use 60mm piping. If you intend not to remove your air conditioner compressor, your downpipe will likely be no bigger than 2.5" in diameter. If that is the case, at no point in your exhaust system will you want the diameter to be smaller than that. Also, if you are to have the system custom made, it would be to your benefit to have it mandrel bent.
Thermal Research and Development makes an exhaust specifically for forced induction Acura's and Honda's. Click
here for more info., part numbers or to contact TR&D.
No matter what diameter you choose, if you decide the exhaust note is too loud, there is a relatively easy and inexpensive remedy. Purchase a good ol' glass pack (I like the Cherry Bombs personally.) from your local automotive parts store. They can be had in diameters of 2.5", 2.75", and 3" and typically cost no more than 20 USD's. Next, weld it in backwards on the B-pipe, the section between the catalytic converter and the muffler. You don't have to put in on backwards but I find that it does not disrupt exhaust flow and also quiets things down a bit more than positioning it the "correct" way.
I HTH and if you have any more questions, don't hesitate to ask.
