One of my return air ductways was made essentially by nailing aluminum sheeting to the bottom of a pair of floor joists and routing that into the main HVAC return. The cross-sectional area of that section of duct is therefore about 8" by 16" (128 sq in). The main return grate in the floor that supplies this ductway is about 30" by 16". To my thinking, the flow rate should be primarily limited by the 8" by 16" ductwork (Q=vA) and I can't see why I need a grate quite that huge to supply it. I can understand minimizing the frictional losses at the entry to the ductwork and that there would be a pressure drop through the ductwork, but having a return grate that size seems like overkill. To my thinking, the return grate should be able to be reduced to a bit larger than 8x16,say 12x16, with the extra size to compensate for friction. Any advice would be appreciated.

Grilles such as your retuen air grille typically have a net free area of about 50 to 60 percent of the total area, so iot stands to reason that the grille should be at least twice as large as the duct that is connected to it. Then, the grille is sometimes made larger to control the velocity of the air moving through it. If the velocity is too high, the grille will be noisy. Those are the reasons your return grille is so much larger than the duct.

Don't mess. =)