In order for your forced air furnace to work properly you have to have the right sizes of duct vents in each room expelling air.
Proper size floor registers.
The overall size of the house the size of the room and the numbers of window in the room all play a part in determining the size of the overall air conditioning unit and the number of registers.
Take out the existing floor register and measure the length width of the opening in inches all registers will be listed in inches.
It is an industry standard to measure the actual hole opening in the floor to get the correct size of floor register or grille that you will need.
The register shown above is a common aluminum adjustable curved blade one way 10 6 ceiling register.
The duct vent size will depend on the size of the furnace and the size of your home so you know how many vents you need in each room.
Registers are the most common type of supply vent.
Air will move straight through registers and grilles.
Registers have the same inlet neck and outlet face size.