Hi!
The general rule is ...make it big!! More volume is better than small volume...you don't have to worry about any exact volumes... just aim for as large as you can make it...preferable as big as the original silencer or bigger. Exhaust opening ...somewhere around 8-10mm!
Steel is better than brass! but brass could ofcourse be used...but is is heavy and could become brittle!
Aluminum is better though, lighter but is not as good in suppressing noice. Harder to weld though!

Regards!
Jan K
Sweden