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.
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.
The register shown above is a common aluminum adjustable curved blade one way 10 6 ceiling register.
Registers are the most common type of supply vent.
Find large and over sized floor registers and floor return grilles in multiple styles and finishes.
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.
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.
Registers have the same inlet neck and outlet face size.
How to measure floor registers and grilles.