Office and warehouse in both Crystal River & New Port Richey ,FL
Depends on the PD of the grille,and what you can afford to deduct from the ESP of the indoor unit,to have enugh ASP for the rest of the duct system.
The above our good ideas for noise and velocity,but you might have to go larger,depending on the rest of the duct system.
Here's an example of a job that required to Carrier Infinity Air Purifiers,due to the pressure drop of the Purifiers,same goes from return grilles and any filters,upsize as needed ,depending on the rest of the duct system.
You need to get the free air area & pressure drop of the grille at various CFMs from the Mfg, then (as Dash stated), you can figure what area is needed based on the ESP of the air handler & the remaining Available Static Pressure (ASP).
Especially with larger tonnage A/Cs; when using filter racks at the rooms, & not the one at the furnace, I believe in over-sizing them, &/or, adding more RA racks.