I agree with Bigchief. A 2.5" system will sound nice and will work well if you decide to upgrade your heads and/or cam.
Unless you have high-flow high rpm engine, a 3" system would be more for show than go.

I have a healthy cam and ported heads and my 2.5" system works well. I seemed to get a bump in mid-range power with the addition of a cross-pipe.