Office and warehouse in both Crystal River & New Port Richey ,FL
The duct system should be sized by ,first doing a "room by room" Man. J,to determine the cfms needed for each room.Then a Man. D,to size the ducts proerly for the cfm required.Not all contractors will be able to do this properly ,so be specific in what you want when calling for an estimate.
With floor grilles ,it's important to have enough "throw" (how high the air will go),in cooling this is critical,as cold air falls.If the cold air doesn't get above 5,6 or 7',the home will not be comforable.
Lot's of "rules of thumb" in this business ,some may work out,most won't.
"If you don't measure,it's just a guess"
Not that familar with Nordyne package units,if you want the best,I'd look for one with a variable speed indoor blower,better overall performance then a standard PSC motor.