typically stock size is 2.25-in's and 3.0-out. the pipe size effects more of the throughput more then sound and you don't want to start changing that without the ability to tune the system correctly. doing so can lead to problems if not done right. if just doing a muffler match it to the existing pipes. if you need more then get a cat-back exhaust system that is built to extract more power and is tuned to the vehicle application. there is a lot of options for different flowmaster mufflers alone so look at them all before choosing.