I don't think it depends massively on horsepower as you are going to get to a point where for example 2.5" would be good enough to flow what you need for a good amount of bhp than say 4". However, I thought it was dependant on things like length and resonance chambers as well as I know from two stroke engines, fine tuning things like the length, width, chamber shape is all very important unless this also doesn't apply to 4 stroke?

There was an application I saw on the internet a while back that was a Java app and basically you could design your own system and could vary a lot of parameters and it would show you how it would effect bhp and torque. I'll try find it.