Every cubic foot of air the blower pushes out has to come back. The return path carries exactly as much air as the supply, and it is routinely built with a fraction of the attention.
Why the return is usually the restriction
It was designed for a smaller blower. A house built when systems moved less air often has one central return sized for that era, and it has since had a larger, higher-airflow system installed on it.
Velocity limits are lower on the return. Around 600–700 feet per minute in a trunk and lower still at a grille, because return grilles are usually in living space at head height where noise is noticed.
Grille free area is not grille size. A grille's louvres block a large part of its face. The free area — typically well under the gross area — is what carries air, and sizing from the outside dimensions overstates capacity substantially.
The filter is usually in the return, and its pressure drop is added to the return side.
Sizing it
The return trunk carries the full system airflow and is sized like a supply trunk at the same friction rate, with the lower velocity limit applied.
Return grilles are sized on free area at a velocity low enough to be quiet — a common design figure is around 500 feet per minute through the free area for a grille in a living space.
The practical consequence is that return grilles are large. A grille that looks oversized to a homeowner is usually correct, and one that looks tidy is usually not.
One return or several
A single central return is cheap and it depends entirely on air being able to travel from every room back to it. Closed doors defeat it.
A return in each bedroom is the best arrangement and the most expensive.
Transfer paths — jump ducts, transfer grilles, undercut doors — are the middle ground, letting a room's air reach the central return with the door closed.
The closed-door problem
A bedroom with a supply register and no return path pressurises when the door is shut. The supply air stops entering because it has nowhere to go, and the rest of the house goes negative and pulls replacement air in through the envelope.
The room is then under-conditioned, the house is leakier than it appears, and in a humid climate the negative pressure draws in outdoor moisture continuously.
A door undercut alone is rarely enough for a room with meaningful airflow. A jump duct or a transfer grille through the wall above the door moves far more air and is not much more work.
Measuring whether the return is adequate
Static pressure on the return side, taken separately from the supply. A return-side reading approaching or exceeding the supply-side reading is a clear indication.
The door test: close a bedroom door with the system running and see whether it pulls or pushes noticeably. A door that wants to slam shut, or that resists opening, is a room with no return path.
A pressure reading in the room with the door closed, which is the quantitative version of the same test.
The cheapest improvement in most houses
Adding return area is frequently the single highest-value change available on an existing duct system.
A second return, or a larger grille on the existing one with the duct behind it enlarged to match, lowers total static pressure, raises airflow to every room, and improves the performance of equipment that is already installed.
It is also usually cheaper than any of the alternatives being considered when a house has an airflow complaint.
What not to do
Do not add return area by cutting a hole into a wall cavity or a floor joist bay and calling it a return duct. Panned joists and stud cavities leak, and in a wall or floor open to a crawl space or an attic they draw in unconditioned air continuously.
Return ducts are ducts. If the path is not sealed sheet metal or proper duct, the system is drawing part of its air from outside the house.