There are many factors to consider when selecting the best-sized furnace for your home. Square footage and construction (insulation and windows) are the main factors but even window exposure to the sun all influence your houses potential heat loss which is why it is best to consult an expert who is trained to evaluate all these factors. Of course, if you are replacing an existing furnace, you can always use its BTU input and efficiency to help determine your needs based upon its successful ability to heat your home.