Im not dissin on Flowmaster or anything and its ultimately your choice, but Ive always like the sound of Magnaflow better. Plus its not as restrictive seeing as how Flowmaster actually creates an amount of backpressure. But for real, to get inlet size, just go measure your tailpipe. And outlet is up to your. Just remember the larger the canister to outlet ratio, the quieter it will be. For example, a 6" diameter canister and a 3" outlet will be a lot quieter versus a 5" canister with a 4" outlet...assuming both canisters are equal length.