I have a two story house (in North Carolina), each floor has it’s own A/C system, the main floor has ducting underneath the floor and the second floor has ducting over the ceiling, the upstairs connects with an “air-handler”; the compressor and heat exchanger is on the ground outside. The main floor has a “gas pack” natural gas fired furnace for heating in winter, gas is cheaper than electricity. Each floor should have its own A/C system and thermostat, bathrooms at night could be cooler than the main floor and during the day the bedrooms could be warmer because no one is there. Larger is not always better for A/C because you want “cleaner air” for health, the larger unit will cool and shut off sooner and less air is cleaned by filters. You can have too much A/C tonnage!