Before any duct is sized, the system has a shape, and the shape determines the longest effective run.
Trunk and branch
A main trunk from the equipment, reducing in size as branches leave it, with each branch serving one or two registers.
The reducing trunk keeps velocity roughly constant along its length as air is dropped off, which keeps friction predictable and the system quiet.
This is the standard residential layout and it suits a house with a linear plan.
Extended plenum
A trunk of constant size the whole way, with branches taken off along it.
Simpler to fabricate, and velocity falls along the trunk as air leaves, which means the last branches see a lower pressure than the first. Acceptable on short runs and increasingly unbalanced on long ones.
Radial
Individual runs from a central plenum directly to each register, with no trunk.
Common in slab-on-grade construction with ducts in the slab or in a crawl, and in small houses. It works well where the equipment is central and the runs are short, and it produces very long individual runs where it is not.
Perimeter loop
A loop of duct around the perimeter of a slab, fed from a central plenum, with registers around it.
Used in cold-climate slab construction because it warms the slab edge. Uncommon elsewhere.
What the layout decides
The longest effective run, which sets the friction rate for the entire system, which sets every duct size.
A layout that puts the equipment at one end of a long house produces a very long run and forces large ducts everywhere. The same house with the equipment centrally located and two trunks running in opposite directions has a much shorter longest run and can use smaller ducts throughout.
That is why the equipment location is a duct design decision rather than a convenience.
Two trunks are usually better than one
Splitting into two trunks from the plenum, running in opposite directions, halves the longest run.
It costs a little more in fittings at the plenum and it is frequently the difference between a system that fits its static pressure budget and one that does not.
Where the equipment should go
Central, to shorten every run.
Inside the conditioned envelope, so the equipment and the ducts closest to it lose nothing.
Where it can be serviced, with clearance for panels and for the filter.
In practice it goes in the attic, the garage or a closet at one end of the house, and the duct system pays for that choice for the life of the building.
Getting the ducts inside the envelope
A duct system entirely within conditioned space loses nothing to leakage or conduction, which removes the largest inefficiency in most American houses.
Ways to achieve it: dropped ceilings or soffits carrying trunk lines, a conditioned attic with the insulation at the roofline, ducts in a conditioned basement or crawl, and floor systems designed to carry ducts between joists.
Each is a design decision at construction and largely unavailable afterwards, which is why it belongs in the conversation at the earliest possible point.
Assessing an existing layout
Trace the longest run from the equipment to the furthest register, counting fittings.
If it is long, if the trunk does not reduce, if there are many sharp elbows, and if the ducts look small — that is the profile of a system that will measure high static pressure, and the fix is usually more return area and better fittings rather than a different layout, because the layout is in the walls.