Doing this properly involves far more than square feet or cubic feet: you need to do a heat loss calculation. Inputs include:
- Outdoor design temperature (how cold it might get)
- Surface area of the building envelope, divided into types (wall, door, window, attic, foundation)
- The U-factor of each type of surface
- The amount of air leakage through the building envelope
The result will be the amount of heating power needed to keep the inside of the house warm enough at the design temperature. Then, you can throw in the efficiencies of the heating system, and end up with your heating requirements.
This is not to say that you must do a heat loss calculation; back-of-the-envelope techniques can give you a ballpark estimate. Do note, though, that it's incredibly easy to over-estimate your heating needs, which can lead to a more expensive and (for oil or gas) less efficient system.