size & "H-pipe"

Velocity is what you want to achieve through the system. The fact is that too large of a pipe diameter will actuall hurt velocity. Remember that we have to blow all the exhaust through a set of pipes nearly the whole length of the car through 2 at least partailly restrictive mufflers. Too big of a pipe causes the velocity to drop. Another thing is in an efficeint exhaust system the exhaust gases need to be kept hot all the way through the system until they exit. Hot exhaust gases move faster. Smaller pipes keep the heat in better & therefore the gasses move faster than a larger diameter pipe would. The natural movement is from a hotter area to a cooler area- heat disipation helps flow. That why racing headers are thermal coated on the inside & the outside. To keep the heat in the gases longer & keep it flowing. What you don't want to happen is the gasses expand & lose their heat before they exit the system, then the velocity stalls & you just have a wall of exhaust that has to be pushed out of the system.
Any body ever run or seen at the track guys running Torque Tubes on their open headers to run quicker? Same theory.
Unless you are running a fire breathing dragon on the street, 2-1/2" exhaust should be plenty in a dual system. If it were a single system then 3" is good.