recommendations for exhaust

[Nate Beck]

Hello all,
  I am getting the exhaust redone on my 85 coupe.  I have no turbo and I am
replacing the manifold and downpipe with one from a 4kq, so it is a 5-3-1
instead of the 5-1-1 that I orginally had.  I also am getting rid of the
resonator and putting just a rear muffler on it.  The question is what size
pipe should I use from the cat on back, I have heard everything from 2" to
2.5".  I wanted to know what the pro's and con's are from each size.

Forgive my inexperience on this matter.  I know that a larger pipe will
decrease back pressure, but what will that do to HP and torque?

Thanks

Nate