Determining the correct size for a small home's gas furnace depends on a number of factors, including the square footage of the home, floor plan and climate. Homes with open floor plans require a different size furnace than those with many rooms.
Homes in colder climates also require larger furnaces. The home's construction should also be considered. Those with brick tend to be better insulated than homes with wood siding, requiring less heating. The amount of insulation in the home, as well as exterior landscaping, also factors into the decision. Standard measurements provide about 25 to 30 BTUs of heat per square foot in a moderate climate.