Dave, Our house is 4,300 sq feet with a two car heated garage. The basement is 1,300 sq feet of that number, so there is 3,000 sq feet above ground. Since the kids no longer live here, we have 600 sq feet of that number closed off. I keep the garage thermostat set at 40 degrees. The house is modern construction with 4x6 walls and only one small area of the house has a vaulted ceiling. There are lots of windows, which is the big item for heat loss.
So, a shorter answer is: 95% of the time we will be heating 2,400 sq feet and the other 5% we will be heating 4,300 sq feet.
Dean