If you actually want this to work use no bigger than 45mm id for road use and 52mm id for track use,depending on header configuration for road an interference 4>2>1 style is best or a 4>1 for circuit track use.
2.25(Two and a quarter Inch) will work on an $8,000 Dollar full race engine otherwise it will go backwards,2 inch (52mm) works for a 2 litre rally car and makes strong power to 8,000 RPM for something like a Ford or Toyota or Datsun L series 2 valve motor,4 valve engines can use bigger sucessfully as I have built many of them and made full exhaust systems and headers all sand bent so perfect bends better than mandrel bends,believe me an A15 will not use over 52mm effectively and will lose torque and throttle response and end up Slow but noisy without a good note to it.
This advise comes from someone who has actually made and tested these things and has a Dyno at work so I actually can measure whether things actually do work rather than guessing and then giving advise even if carefully considered.
Unfortunately many these days go more for "Bling" over substance canon exhaust systems being a good case in point,almost invariably these make the car much slower,but the noise output makes people driving them think it is quick,plus nobody wants to admit they screwed up and wasted their money.