Stock

That kind of thing doesn't impress anyone except other kiddies and, as has been stated, does not really do much of anything unless your engine has been built to take advantage of additional flow capacity.

minimum 12 inches

The only people you will get attention from with a larger exhaust system are your pissed off neighbors and the police.

Unless the engine is built and you have a turbo normal size pipes are fine.

Unless your boosting don’t go over 2.5 for the pipe. Bigger is not better with less back pressure you’ll lose bottom end performance 

I ran cammed out V8 engines for years, even those only needed 3" duals, 4" if a single exhaust.

You're forgetting the rest of the exhaust will be tiny (2"?) & that muffler will do nothing but make it sound annoying, it's the equivalent of just chopping off your muffler & welding on a paint can.

If youre talking about tip size, is just aesthetics, whatever look you like better go with that. If youre talking about the entire muffler, its not going to change much other than sound. Typically your exhaust manifold & cat creates enough back pressuse so it wont change how the engine runs. What is your goal?