Furnace strength is measured by BTUs, a unit of heat output. The BTU rating needed for a furnace depends on the size of the house. You should carefully calculate your home's BTU requirement before purchasing a furnace.
British Thermal Unit

BTU is short for British Thermal Unit. One BTU is the amount of heat it takes to raise the temperature of a pound of water at maximum density by a single degree Fahrenheit — about 1,044 joules.
Furnace Sizing

Buying a furnace of the correct BTU rating is very important. An overly powerful furnace will "short cycle," or turn on and off repeatedly, wasting energy; while a weak furnace will be unable to heat your home.
Calculating BTUs

To find the right BTU level for your new furnace, you simply have to measure the dimensions of your house and crunch the numbers. You can do the arithmetic on your own — multiplying your square footage by 40 in northern states, 35 further south, and 25 along the West Coast — or use an online automatic calculator for more accurate results (see the link in this article's Resource section).