Measure the interior dimensions of your home, finding both the length and width for the entire building. Measure a typical room's height from floor to ceiling as well. If you have a two-story home, add the ceiling heights for the two levels together.
Determine whether your house has good, average or poor insulation. One test is to hold your hand against an exterior wall during the winter months -- if it feels cold, you likely have poor insulation. Drafts around outlets are another good indicator of inferior insulation quality. Default to "average" if unsure. Consider your home to be in a temperate climate if it is located in USDA Hardiness Zone 6 or higher and in a cold region for Zone 5 and below.
Enter your collected data into an online BTU calculator, such as those available on Hearth.com and HermanNelson.com. Fill in each field then press the "Calculate" button to compute the hourly BTU heat requirements for your home. Purchase a furnace with a rating as close to this result as possible.