Andy mentioned Bill Pentz' worksheet and Ole mentioned the 7" trunk. The worksheet is important because ultimately you are looking for effective vertical and horizontal velocities for moving the kind of dust you expect. The engineering of my system suggested a 7" main trunk but I could see higher CFM having pushed me toward 8" or larger. My inlet was 8" and I reduced with a custom 8" -> 7" transition reducer.
I used Spiral Mfg. 20 gauge which has worked great, but 26 should likely be adequate and lighter and cheaper.
"the mechanic that would perfect his work must first sharpen his tools.” Confucius