How I approach duct sizing has nothing to do do with size vs cfm. It depends upon total external static pressure and what the equipment can deliver at a given ESP. Other factors are noise, distance, main branch, sub branch or final branch.
If you have a 4 ton unit at 20x20 filter grille is going to be way undersized. I don't think a 4 ton unit can even deliver 2600 CFM. Min and max cfm's should be determined by a professional with a proper load analysis as they vary from climactic zones.
Politicians need to be changed like diapers, and for the same reason.
Mark Twain