Return air size

Thanks for the info. All four return vents I mentioned above are 6”x14” off of an 8”x12” duct. The main return at issue can be cut anywhere between two 24” oc joists. The duct leading to sheet metal between the joists is 8”x9”. The main return grille that the mechanic put it initially is 8”x24”. I’d like to go as small as I can due to the location. My furnace is 80,000 btu and the conditioned area is about 2000 sq ft including basement. I’m not sure if that’s enough info to make a suggestion...but given that the duct leading to the vent is 8”x9”at its smallest point, am I right to think I can get a floor vent close to that size?