I was prepared to go 2.25" (no resonator) based on feedback here, and if not for the shop's encouragement to go 2.5" (no resonator), which I had assumed would be louder, but I cannot imagine it being much quieter.

Here's just a little handy hint, when putting the exhaust gasket on, grease both sides with a moly grease, this will stop it from sticking to the head or exhaust and destroying it the first time you take the pipes off!

As for pipe size I've been told by a well respected mentor that on N/A cars the exhaust works like a siphon. There is an ideal diameter where the air flowing out the exhaust helps pull the air behind it out. Too big and this won't happen.

On a near standard 6 cylinder car 2inch is plenty. 2 1/4 is normally cheap for mufflers etc, 2 1/2 sounds too big to me.

If you want to quieten it down put in a triple flow muffler. Don't use straight thru's.