There are a few aspects that influence the size of the furnace, namely: The age of your residence The size of your house Number of windows and doors Insulation Airflow To be on the safe side as far as getting a suitable furnace is concerned, a home visit by a professional is recommended. The expert will evaluate the residence and calculate airflow and heat loss. When you choose the wrong furnace size, your bills might be on the higher side due to inadequate warming. Also, the lifespan of the furnace may reduce drastically.