No muffler expert here but I have made a few custom ones over the years. All steel by the way as they were used on airplanes that needed nose weight. Simple rules I follow are to give the muffler as much volume as possible. That means make the can as large as possible! A large can also helps reduce the exhaust volume a tiny bit by slightly cooling the exhaust gases. I prefer a large diameter, long exhaust tube or tubes as these tend to produce what seems like a lower exhaust note. Smaller tubes produce a sharp cracking sound. I doubt the actual noise level changes much between the different tube sizes but the perceived sound level certainly changes. I've never used a baffle in the mufflers as I wouldn't know how to design them and would probably do more bad than good by installing them.