There is a "load calculation" formula that involves far more than square footage.
Any professional HVAC company should be able to perform one free to properly size the system(s) for your home.
Too small, and it simply won't do the job heating or cooling. Too large, and it will cool down too quickly before dehumidifying, causing mold and mildew problems.
Get a proper load calculation performed before sizing and planning the system(s).