What size do most manufacturers use when making single in/single out or single in/dual out?

I currently have a true dual setup with a magnaflow dual in/out using 2.25" pipping. Power is pretty good, but only in the upper rpm's. I was thinking of switching to a new setup. Highflow cats into new Y pipe---from here i dont know what size pipping to use. I got a 4.6 and i have a 4x4 so I want to try and keep torque at the bottom.

What do some recommend? I want to keep my tail pipes that go out the corners and are 2.25" pipping.