To choose the right size of forced-air furnace requires that you do a load calculation for your home. A load calculation takes into account a number of variables that affect the air flow in a home, from the size of the home to the number of windows and its location.
There are two types of load calculations. A block calculation is used for single-story homes with existing ductwork, while a room-to-room calculation is used for multi-story homes or homes without existing HVAC equipment. Both types of calculations can be run online for free from many home improvement or HVAC contractor sites.
The block calculation uses the overall size of the home as a base, then takes into account the number of windows and doors in the home, foundation type, roof type, insulation values of the materials used in the building and the desired temperature of the home. Once the calculation is run, most sites provide a recommended furnace size. The room-to-room calculators use the same variables, except each room's measurements are placed into the calculation instead of the overall home size. As a result, not only is the proper furnace size recommended, but duct locations and damper settings as well, to ensure even temperatures throughout the home.