Instead of sizing the A/C and heat to the house how about designing the house to work with the HVAC? How about a building code that limits HVAC units to a certain size per sq ft and have the builder/insulator design/fix the house so that it will cool properly? Let the builder decide if they would rather get rid of the black roof, huge west windows, or insulate more/seal properly. This also would force the units to be installed properly instead of oversizing to cover up installation problems. For the heat side the furnace will almost always be oversized just to get the blower capacity needed. I've never seen a situation where the the house cooled fine but the furnace couldn't handle the heating load.
In our area the limit could be something like this:
New construction: 1,000 sqft per ton (this isn't unreasonable in our area for properly insulated/sealed house)
Existing homes built after 1990: 800sqft per ton
Homes built 1970 - 1990: 650 sq ft per ton
Homes pre-1970: 500sq ft per ton.
Lets start fixing houses and correcting installation issues instead of just sticking in a bigger A/C to cool things off.