65, think your idea of making it bigger in diam if built longer is good. The thought being not to introduce more back pressure (then stock) with the added length. The stinger diameter of the pipe may help you decide if a bigger diam is needed (backpressure wise), if it's relatively small in comparison I doubt you'd need a bigger core.
One of you limiting factors will be the availability of different size perforated core tubing. You can do the math and theory and come up with numbers, but in the end you make a decision based on whats available for stock (even the big boys are limited by this). This assumes, of course, your building a glass pack style? If your building a baffle design calculating back pressure can get pretty tricky.
Thoughts: Build it to accept a forest service approved tip if you plan to be where it's required. Make it as serviceable as possible for repacking. If you really want to get into it I might have some design software I could share with you. It's more for predicting sound attenuation at different frequency's than actual "how to". I was working on a pressure rebounding exhaust system based on silencing and vibration reduction technology used in high pressure gas lines. There's a couple company's that have picked up on it now so I don't mess with it anymore.