As stated it depends on the house. Insulation, outside design temp, indoor temp, windows, air infiltration and even which direction the house faces all are factors. Size the heat pump for cooling though, not heating. A two stage two ton is a perfect match for most homes that size here.
Yes Tootsie, your query doesnt say what region of the country you are in or the type of house and open windows/doors etc. I think you can do 1.5 tons generally and be efficient. Checkout the EPA and DOE govt specs. Article is a little overkill but you will learn and benefit comprehensively http://www.homeenergy.org/archive/he...95/950509.html