BB1; Im sure there are ways to calculate all this, and for sure if you are wanting a tuned setup there is plenty of info out there for calculating tuned pipe design.
Ive been working on mufflers and headers for myself and some friends. I am using .875 ; .049 wall tubing on headers because I started working on a BME 102 and that was close to the size of the exaust port, nothing scientific. I have also been able to shape it into the size needed for some other motors that do not have a round exaust port. The larger single cylinder motors would probably need something a little larger.
As for volume I would say get as much as possible, within reason of course :stupid: Less volume = more noise. Im sure there is probably a point where its not worth the effort or there just isnt room for anything bigger.

As soon as I get some plane building done I plan on doing some more muffler testing.
Want to run some tests for me?? Do you have a DB meter??
I would be glad to fix you up with something, not free but very reasonable if you could give me some feedback as to sound and performance.

Im not sure if a Moose Can will hold up to a gasser, they are awful thin.

Adam