i have done a lot of research into what works for our trucks. If you are looking at a true dual setup (ie, dual pipes all the way from the motor till it exits), then you want to stop at 2.5" pipes. Yes, you can go bigger and net more power, but it is also at the cost of low end torque. This is where you have to decide how much time you spend at say 2,000 RPM in relation to 4,000 RPM. Also keep in mind that depending on how many cats your truck came with (some came with 2, others with 4) and also the series muffler you are running (very restrictive, semi restrictive, free flowing), this can also affect how much power you are going to get out. So, a simple straight forward "this is the ideal setup" is not realistic.
Now, with all that being said, I have a true dual setup on Nukie which is a 2.5" pipe setup, no cross over pipe, running into a Flowmaster 70 series DI/DO muffler. By no means is it one of those systems that will catch your attention due to the rumble it makes. But then, I built the exhaust system around making maximum low end torque. The way that I use my truck, I need that 1,500 RPM grunt, not the 5,000 RPM scream. But to each their own.
Please keep in mind that when you supercharging/turboing a motor, a lot of the exhaust stuff that I mentioned goes out the window as you now have other factors playing into it.