Looking at the existing heater is NOT the answer to finding the right size. On the other hand, if the home is comfortable, that can be a clue.
The "official" method is Manual J, published by the Air Conditioning Contractors of America ( ACCA). http://www.energysavers.gov/your_hom.../mytopic=12340
Most contractors these days have that on a laptop, but of course you can do it "the old fashioned way". But if you don't do a thorough load and heatloss calculation, you are just 'winging' it.