I have a Bryant Unipart #395A residential gas-fired forced-air furnace. It has a 12"x40" floor return air opening. I can't find a grill to fit that opening (presently 3 overlapping smaller ones are used) but would like to find one that fits. I was told that the opening is bigger than it needs to be. What is the normal return air size requirement so I can reduce the opening and find a smaller grill?