For your new system to function, it needs to be sized for your home. Your contractor will perform a manual J calculation, also called a load calculation. This considers various factors about your home. They include issues related to heat loss, heat gain, and efficiency. Measurements are taken. Construction and features of the house are considered. This info will help your contractor determine the capacity needed for a new system. There are other needs to facilitate the new system install. These include ductwork installation or sealing to improve energy efficiency.
If a contractor offers a quote over the phone, you cannot know that the system is the right size for your home. Before choosing a contractor, ask if a load calculation was performed. Ask why the contractor has recommended the specific model and heating or cooling equipment. Is its capacity right for your home?