The short answer is you really should get a contractor to determine the proper size of HVAC equipment needed. To calculate the proper size of heating and cooling equipment for your home requires use of an engineering procedure such as ACCA/ANSI Manual J. Using short-cuts or rules-of-thumb typically results in equipment that is incorrectly sized for your home. Your contractor should determine the right size for your HVAC equipment by using ACCA/ANSI Manual J or an equivalent sizing calculation tool that takes into account specific information about your home. Homeowners who are interested in sizing calculations can purchase the procedures from Air Conditioning Contractors of America (ACCA) (Manual J-8 Abridged Edition is $64).