I live in Washington DC, 2400 sq.ft. t hree story house, basement, main and first floor. There is one central return duct that goes straight up from the system in the basement to the ceiling of the first floor.

It has two return air vents. One is on main floor in the living room other vent is on ceiling on the first floor above stairs. Both are same size 24x12.

Problem is that return vent on the main floor is so dominant that it sucks almost all the air (sucking power of the upper floor return vent is so small that it won’t hold plastic bag when system is running) so during the summer time hot air stays trap upstairs, while AC is sucking and cooling already cold air from main level. I have already closed half of the vent on the main floor and that helped a lot!

So I want to close the vent on the main floor completely and make one more return vent right next to the existing on the first floor. Because I am reading a lot of this site about under/over sized return vent, I what to make sure that my unit will have enough air supply. So I am asking for advice in the size of my new return vent. Should I make it same size or..
Return duct size is different on every level, in the basement I use 10x20 air filter, located right next to furnace , duct that goes straight up is 15x15 and above first floor ceiling is 10x15.
All doors in the house are cut for air circulation and upper floor vent location is at the top of the staircases. No doors between floors.

Thanks a lot!