A/C size really depends on the area where one lives. A unit that keeps a comfortable temperature and humidity in one area will be almost useless in another.
Houston, NOLA, and Florida are not just hot, but humid, so a swamp cooler is useless. Removing humidity is important as well.
Of course, too large an A/C may not remove the humidity from the air, causing the air to get clammy.
Another factor is that a larger A/C has more locked rotor amps. A 11k BTU A/C can start and run from a Honda 2000 watt generator. A 13.5k BTU model is iffish. A 15k BTU A/C might not be startable even with a 3000 watt A/C and would require a pair of 2000 watt Hondas.
I live in Austin, so take this for what you will. A "B", I'd use a 13.5k BTU A/C like the Coleman that can run from a 2500 watt generator. If I had a "C" with a slide, I'd go with a 15k BTU model. With slides and longer than 28-30 feet, I'd go with two A/Cs. Otherwise the rig may be uninhabitable from May-September.