William, as Brian mentioned, the system should be engineered (planned) for the amount of air flow, type of duct and length of run. One point I wanted to make is that IF there is only one return grill, the size of the duct will remain constant all the way to the unit. IF there are multiple grills, then the ducts will be increased at each wye to allow for more volume. The purpose is to insure each grill is pulling air in rather than everything drawing from one grill if the same size duct is used throughout, dampers could also be used to control airflow, especially if it is a 2-story house with heat and air that needs seasonal adjustment.
Lots of variables can come into play on ductwork design.

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Jim Luttrall
Mr. Inspector.net, Inc.
Allen, Texas 75002