Are you getting the Stromung mid-pipe? I guess I'd have to say 2.25" then. Matching the size of the mid-pipe would be your best way to go. Going any larger isn't gonna help you out. A 2.25" inlet will give you a wide variety of mufflers to choose from.
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I'd say 2-1/4". but if you have big future plans for your car, such as nitrous, turbo anything bigger than that will do but nothing more than 3" it all depends on what and how much power.
somewhere along the lines of that, but i could be wrong.