For a stockish ej20t with a factory ball bearing turbo- how big is too big for the exhaust from the turbo back?

I'm thinking 2.5 inch, but most aftermarket wrx systems seem to be 3 inch or bigger. Pipe will be as short as possible (hopefully only about 4 feet including cat and muffler)

I doubt you'd notice any difference on a mild setup between 2.5 and 3, esp since the run will be short and have minimal bends. I got 2.5 on mine.

I think I remember hearing somwhere that they use 2.5 for mandrel bends but for press bends (is that what they are called?) they step up to 3 to compensate for the lack of flow.
Simon

We'll just be welding straights and prebent sections (doughnuts) of stainless. sounds like 3 inch is the go.

Yeah press bends crush and so the size reduces, so yeah 3" probably goes down to 2.5 say. Your doughnuts should be the same diameter all the way through the bend (like mandrel bends).
Actually, just having a look in the "21st century performance" book, they did some flow tests on 2.5" pipe... on a 90deg bend, the straight bit of pipe flowed 161cfm, mandrel bend 90deg 160cfm, press pend 90deg 156cfm. The difference between press and mandrel increase the bigger the bend. The 45deg there wasn't much difference.