The size is typically measured in tons or BTUs (British Thermal Units) per hour. A common rule of thumb is 20 BTUs per square foot of living space. However, considerations like climate, ceiling height, insulation, number of windows, and the home's orientation also play significant roles. It's best to have a professional HVAC technician conduct a load calculation using the Manual J method to ensure the unit's size matches your home's specific cooling requirements for optimal efficiency and comfort.