Stock-ish 5.7 Build...2.5" or 3" Exhaust?

about 2.2 cfm of flow per horsepower will get you 99 percent of the open exhaust power.
Straight pipe flows 115 cfm per square inch. A 2.5 inch pipe flows 560 cfm, but there will be some flow loss for bends and total length. Then you need to look at the mufflers. The pipes can't flow anymore than what will go through the mufflers, so they have to be matched to at least the minimum cfm you need for least power loss. According to one chart I looked at, if you only have 1.75 cfm per hp, then the loss will be about five percent of total power. So a 400 hp motor will lose 20 hp on the top end, and less in the mid range. Be sure and use a balance tube in the system, this will help with sound and total output as well.
I just installed a 2.5 inch system on my 65 street dart, for the next motor which should be 450 hp. The mufflers I used are 17733 Dynomax, which flow about 410 per side for about a max 4 or 5 percent loss in the 475 hp range. I can handle 22 less hp on the top end on a car I don't race.