2 1/2 is the most popular but I think someone had made a 3. I think speculation was that 3 is too big because the turbo outlet is only 2 1/4 and jumping to 3 that quick just adds turbulence so again, no benefit.
You'd need a nasty turbo and motor to need 3.5" or 4" exhaust. I worked at Magnaflow for years when diesels got popular. A 4" downpipe was overkill on the big V8's, unless they were hot-rodded. Saw the same on turbo import cars. They like exhaust, but too big would only allow the turbo a temporary spike, they couldn't maintain the boost and power. We avoided going bigger and bigger for awhile, but the market wants what the market wants. Great times working there