A Note About Capacities:
Gas furnaces are generally rated by "input" in Btus per hour (Btuh). A furnace rated at 100,000 Btuh that is 80% efficient (80% AFUE) will have an output of 80,000 Btuh. In other words, 80% of the total heat produced by burning the gas will be in the form of usable heat to warm your home. The other 20% is exhausted from your house along with the flue products.
By the same token, a 100,000 Btuh furnace that is 90% efficient only sends 10% of the total heat out the chimney - thus burning less gas to get the same results and reducing your gas heating costs.
A Note About Oil Furnaces:
Many oil furnaces are dual rated. That is, they are listed with two different heating capacities.
A Note About Air Conditioning:
You've heard the saying "It's not the heat, it's the humidity." Air conditioning systems do more that just cool your home - they remove moisture. The more humid it is outside, the harder an air conditioner has to work. But does that mean that if you get a bigger unit, it will work better? NO. An air conditioning system that is too large will neither cool nor dehumidify properly, and the result will be an uncomfortable, clammy home.