general rule...17 btu/sq ft.
when sizing a heat pump option if the structure has above average heat loss or is not utilizing ac...up sizing up to 25% on the first calculation can be done.
eg. 2000sqft x 17+3400 btu (3 ton) x 1.25 = 42500 (3,.5 ton)

If you go with a two stage unit, ac problems from over sizing wont come into play as much,you can give the home owner a larger unit more closely matched to their heat loss and the system wont utilize the back up heat as much if at all.
Usually a retro fit from another heat source will have questionable duct sizing which will give a noisy/higher velocity air flow from the needs of the 400 cfm/ton for the heat pump, when a two stage is used it will run on first stage most times at a reduced fan speed giving a quieter air distribution system with out costly sheet metal alterations.