Jerry- How much bigger diameter pipe do you need as you go up in HP? Is there a general rule of thumb

It is more complicated than how much diameter do you need for a certain amount of HP.  The number of bends, the quality of the bends, the length of the stinger, mid-pipe and silencer, and the hole size and spacing of the holes in the silencer core all affect the flow rate of the exhaust gas as it leaves the pipe.  Every engine design has it its own expectations as far as optimum exhaust pipe restriction.  Dyno testing and experimentation is the rule of thumb I use.