Anyone out there have a rule of thumb, or a link to a chart, for rectangular duct outside dimensions? I am using 28" x 28" Gauge 24 and 12" x 10" Gauge 26 galvanized steel. - Jinx 
I’m going to bet that what you’re looking for is within a SMACNA publication, but you can’t get it free.
What size your duct is depends on how much air you are moving. There are rule of thumb velocities you attempt to design for, but no rule of thumb for duct sizes.
Unless you are talking about small ductwork in your house. Then the rule of thumb is to make it whatever size fits in the opening you have available (between the joists, inside the studs, etc.)
for the most part, all duct is custom made. In general, if you are specifying duct sizes, it’s best to vary only one dimension in a transition. That is, if you reduce the horizontal dimension, leave the vertical dimension constant. The transitions are much easier to make this way.