Gas and Oil Home Heating Furnaces - Air intake register question

What size air intake register is required for a 1375sq ft home. The home we bought has two registers side by side. The original register was 9 x 21, which we enlarged to 12 x 24 and at some point, someone added an additional register, right next to the original, we assume to increase air flow to assist the motor?? The bathtub also shares this wall so we are not sure if the hole was used to access a leaky tub at some point. Are question is...
Do we need both registers or is the new 12 x 24 sufficient for our HVAC system?

Id say its not the size of the home that counts here . Its more in the size of the furnace and AC unit that you have as to what size the duct work and grills should be in a home.Just a rough idea here, dont say its just right for you is 60" to 80" sq " per ton for AC unit So a 3 ton at 60sq" a ton would need a duct of 180sq" back to the unit for a return and dont forget the cold air grill .You lose about 15% of its size so it has to be bigger.;) ED