You're not going to see very much of a gain over 2 inches unless you have done some pretty serious work to your engine. Anything over 3 inches is overkill and will just give you that fart sound everyone hates.
Or maybe you're talking about exhuast outlets. If so, you'll get the nickname "Grapefruit Shooter" pretty soon.
My bad, I should have specified: I'm looking at Magnaflow aluminized universal-fit round body mufflers and I was wondering what difference it would make if I put a 4", 6", or 7" AROUND muffler on my car. I wasn't talking about the tip. Hehehehehehe, 7" would be pretty loud- loud as it scraped on the ground behind my car.
Ohhhh.. I assume the larger the quieter and deeper. I like the dynomax pretty well. It'll give you a deep and throaty sound for $60. Mine's more mellow now because of the 20" glasspack and the dual 2" resonated tip.
I like my setup because you can barely hear the exhaust with the windows closed and cruisin at 60mph. My old setup with a 2-1/4" exhaust and a tsudo rice-boy muffler was sooo loud. Just cruisin you could hear it droning througout the car and everytime you went under a bridge you felt it.
Dynomax is awesome! I've checked their flow ratings of their mufflers and on their 2" ultraflow 17514 welded muffler it flows 540cfm. Compared to edelbrock and flow master there's about 35-60% more flow. The stainless steel was something like 425cfm. I'm not too sure between the import mufflers because most of them don't have flow charts that come with them.
The Ultraflow's are straight through with preforated cores. The ultraflow 17514 is a 2" ID inlet / 2" ID outlet and is an oval 4-1/2"x9-3/4" It is has an offset/center setup. You can use the offset for the front side towards the front of car so it's easier to get the pipe to it (offset towards the passenger side of car) and then out the back it'll be centered.
You can get this muffler from www.summitracing.com Goin this route will allow you to spend your extra money on a custom 2" mandrel exhaust and maybe an intake or something. (I've gotta Ractive heat shielded intake that I'm wanting to get rid of... you want it for $10-15 + shipin? you'll need a MAF adapter plate.)