Whether remodeling an existing residence or building a new structure, it's critical that the HVAC system be properly sized. In the case of a heat pump, a unit that is too small will fail to heat or cool the air sufficiently, while an oversized system will reach the desired temperature too fast, shutting itself off before it can fully function as a dehumidifier and opening the home up to potential moisture problems.
Gather the Measurements Required for Load Calculation
The easiest step is the first one: calculate the square footage to be conditioned. Either take direct measurements of an existing house or work from blueprints for new construction. Do not trust measurements recorded on auditors' sites or real estate listings, as they may not be up to date.
Look at the insulation in the attic and basement. If kraft-faced batts of insulation the R value will be printed on the brown-paper backing. If you have blown-in or spray foam insulation, you'll have to measure the thickness and do you best to estimate the R value.
Windows and doors are major sources of heat loss. How big are the windows in your home? Are they single paned or insulated? What direction do they face and how much shade do they receive?
Add all the information you've gathered into the load calculation software of your choice to see what the suggested size is for your home. (Depending on the software you use, it may ask for more or less information than is mentioned above.)