How many cubic feet will a 100000 BTU heater heat?

Furnaces and central AC units are sized according to the homes heat loss in btuh ( for heat) and heat gain also in btuh (for cooling). If you or your contractor have opted not to do this calculation a rule of thumb guesstimate would indicate that a furnace rated at 100K output (not input) would be satisfactory for a home with average insulation, Windows (#, efficiency and the direction many of them face) etc in the neighborhood 2000 square (not cubic) feet, plus or minus which means you may wind up a little oversized or a little short. Since you dont know what the home is actually losing or gaining. Do a google search on HVACCALC to see more on what Im talking about. Lastly, you dont say where the house is, it matters since it will heat a much larger home in Miami than it would in Minneapolis.