Load calculations do look impressive, and I tip my hat to all those who can do them. However, given my (limited) experience in this industry, I am almost in the "size doesn't matter" camp. A good example of this is my own house.
I've never done a manual J on my house. But the equipment that's there is a 100,000 btu (80% afue) 3 ton (10 seer) outdoor packaged unit. When I go back over my gas bills for the most unforgiving winter months, I used slightly over 25,000 cubic feet of natural gas. This means that I theoretically could have heated my house with a 35,000 btu furnace. Sure, it would have run nonstop, but that wouldn't have been without its advantages. For example, with the continuous blower providing "constant air circulation", the heating of the house would have been extremely balanced.
But, if I were the contractor installing the unit on my house, I would have never put in a 35,000 btu furnace. Why? Well, for one thing, the house has twelve 6 inch heat runs. This is usually a good indication that the duct system can provide 1200 cfm of airflow. A typical furnace that delivers that kind of airflow is a 75-100,000 btu furnace. And I strongly suspect that if someone sneaked over to my house in the middle of the night and swapped out my old furnace for a 75,000, I would scarcely be able to tell the difference. To put this into perspective, a 75,000 btu furnace runs 2 hours to do the same job a 100,000 btu furnace does in an hour and a half. But if they swapped my furnace for a 35,000, I would probably notice that my furnace is running nonstop, and I can't turn it down when I'm away, and expect the house to heat up quickly when I return.
In sum, a rule of thumb I use for sizing equipment is this: I count the number of heat runs that are in the house. I turn on the fan to the existing furnace to check that no runs are starved. If no runs are starved, and if all runs are 6 inches, I multiply the number of runs by 100 to get my cfms. I then select a furnace that delivers that many cfms, and propose it to the customer. If the customer seems unhappy with that selection, and wants a bigger furnace, I tell them they also need a ductwork upgrade (furnaces that are oversized for their ductwork tend to fail prematurely). Usually, they just take the furnace I suggest.
So far, this method has yet to fail me.