When replacing heating and cooling (HVAC) equipment, bigger doesn't always mean better. If the unit is too large for your home, you will be less comfortable and might actually have higher utility bills. For example, an oversized central air conditioner will operate in short run cycles, not allowing the unit to reach efficient operation and remove humidity from the air resulting in an uncomfortable home.
Your contractor should determine the right size for your heating and cooling equipment by using ACCA/ANSI Manual J or an equivalent sizing calculation tool that takes into account specific information about your home. Visit ENERGY STAR Heating and Cooling for more information.