Your ac contractor or engineer will determine the correct size.
really depends on the size of the unit but if your not sure i would go with a 20x20 or a 20x25 filterback grill..
Air return duct and filter grille size is dictated by the size of the unit (CFM) not the size of the house.
The best rule of thumb I have found is: Length X Width / 144 * 288 = Max. CFM I have used this formula to size many HVAC systems. It has always worked. For example, a 5 ton unit should have a 30x36 grille(2-18X30 filters). (30x36)/144*288=2160 max cfm. Return Air should be sized at 200 square inches per ton according to AirMate and other filter grille manufacturers to achieve 2.0 static pressure. The Return Air Duct should have an unobstructed 6 square inches per 1000 BTU of cooling.
a 4 ton unit
A 16" round flex duct will accommodate up to a 3.5 ton unit
quick down and dirty way is the size of your unit (example..4 tons) X 144= the square inches needed for the return vent size or the longer way Max air flow from unit (rated size) / 2.08 cfm/sq in= filter area needed
The grill if I recall correctly is one unit. What I mean by one unit is that the grill housing goes around the headlights / around the whole front of the vehicle. I have a 1998 Ford Expedition XLT. I ended up cutting my grill out of the housing and installing an aftermarket one in its place.
The size of any unit is, by definition, 1.
70 amp
There should be appx 15*-20* difference between return and supply air temps measured at the unit (not the registers).
it should be well defined it should be of suitable size it should be internationally accepted it should be easily reproducable it should not be affected by the environmental conditions
You should not have to calculate the watts of the unit. All the information that you need will be on the nameplate of the unit. It is this information that electricians use to calculate the conductor size and breaker to supply the power to the unit.