3" diameter pipe is the biggest that can fit in the fenderwell hole... you can use 2.5" and still get good clearance with the battery and hump next to the fenderwell. Might as well get the biggest one to maximize whatever gains you can get from the intake.
Well, here's the deal if you want the best air intake. I have a 95 Sable 3.8, and i wanted to make it ram air, which would be the absolute best because i can gain back my lost pressure in my intake manifold, and gain more performance. But because of the way the crammed everything in there so tight, i can't do it without getting a different hood. But anyway, 3 inch PVC pipe, no angles, no bends at all, that will just completely throw your air off and you'll lose air velocity and pressure. So just go straight out from the MAF. That's what i did, and that's the best i can get without major body changes. Oh and i bored out the MAF to 75 mm too oh, and the fenderwall hole, unless you intend to do it ram air, and completely make it perfect, with all of your eqautions and exact numbers of pressure and such, don't waste your time, you won't get much out of it, especially since you have to bend it all over.