Load calculations are the correct industry accepted way of sizing equipment according to the temperature ranges in your area. The building volume, areas of floors, walls, windows, doors, aux heat sources, number of people, direction of exposure are figured. When your done they give requirements in BTU/HR, CFM in heating and cooling for the structure.
If you do a whole house load calculation it will tell you the equipment requirement and CFM for the whole house. If you do a full blown calculation listing each room it will give you the BTU/HR and CFM requirements for each room. You use the full blown room-by-room calculations to size the air distrubition system.
No matter what others say or believe, guessing by some BTU per square foot to come up with a size is not accepted by true industry professionals.