2.5 inch or 3 inch
Moparwedden, a large high flow exhaust pipe does not mandate that it be loud. You can use big pipes, oval if required for clearance or trickness=P~, and make it as quiet or as loud as you like. I'm assuming that you have issues up there with sound level laws...you can use a quieter muff with the 3" or greater pipe and still have tons of flow. Best of both worlds.
