Change the stock mufflers need to know size to buy

Unless you have modified the engine in your new car and thereby voided the warranties, the existing exhaust system is adequately sized. If you want louder mufflers you should be able to replace those safely without endangering your warranty coverage, but read the fine print in your 3/36 bumper to bumper and 5/50 power-train warranties. Anything behind the catalytic converters is generally fair game (cat-back conversion) but again read your warranties. I have about 17 months left on the power-train warrantee on my 2010 Challenger before I will even think about any modifications.