We keep our sliders open 24x7 and use our mini split to also cool our family room!

We have a 15 by 28 enclosed with double pane windows last fall and installed a 1.5 ton unit Mitsubishi 1.5 Ton GL18 20.5 SEER system. Note that the window installer recommended that we wait a year or two and experience just with the sliding doors open. At my age , I decided to invest now to enjoy the benefit. Glad I did because I can now enjoy all of the time. Without the unit on, the temp can get up to mid 80's on a 90 F plus day. I suppose I could open bedroom sliders and LR sliders to cool off more. Note we have 3 overhead fans in the sunroom and installed the unit on the house wall nearest the window side of living area. The unit has directional vanes to force airflow in proper manner and ceiling fans provide even more mixing and cooling.

Found dollar variation for unit and install not that significant to choose smaller size or lesser model (not worth any compromise after undergoing the total enclosure time, effort and cost experience). Not sure the real advantage of ceiling cassette. In my case, I finished the ceiling in Knotty Pine and with the fans, felt that I did not want to center the unit in the ceiling and thus get airflow, onto our lounging area. Also, concerned some what over any maintenance issue in the future. i.e. condensation drain line maintenance etc.

PM me if you wish.

It’s not the cassette that is so heavy. It’s the music on it.

Get a bid from Daniels HVAC in Inverness. We got three bids they were the best.

There are 2 pieces to sizing.
Obtaining desired temperature is only half of the story. Run time short or long will cool temperature. Longer runtime will improve the evenness because more time to circulate. Short runtime starts to short cycle the compressor, leading to unnecessary wear.
You must also reduce humidity to feel comfortable. That happens from run time. Longer run times from a smaller unit will drop humidity most effectively. Short runtimes from an oversized unit will leave you cool and sticky. That's awful.
This is not like a car where a big engine will get things done quicker. Don't be tempted to "go a little bigger" because it's not a lot of more money. It does not scale like that. You will pay more for worse results.

How big of AC unit
 

It doesn't matter the model of home - it's how many square feet. You can find some good information on the internets. I found -
A common rule of thumb is to assume that a standard HVAC unit can cool around 400 square feet of residential space per ton of cooling capacity. When calculating the power you need to cool your home, you can assign 12,000BTUs that need to be cooled per 400 square feet of property. So determine the square feet of you area. If answer comes to over one ton, I would round up to 1.5 as I suspect the area may not be well insulated causing heat to build up more rapidly than in "regular" rooms. Also, I assume the calculation above is for a standard eight foot ceiling. I did not see any calculation for cooling needed for cubic feet which would have been more helpful.

Multiply your cubic foot measurement by 1,027 to convert to British thermal units. For example, a 50-cubic foot room multiplied by 1,027 equals 51,350 British thermal units.
Step 3

Divide your British thermal units number by 12,000 to convert to tons. For example, a room that requires 513,500 British thermal units per hour to cool would require a 4.3 ton air conditioning system.

Don't forget the baffles so that the soffits still draft

None of that thinking applies to these mini-splits. My Mitsubishi has a variable speed compressor and can ramp down to as low as 25% of the nominal capacity. This means it runs all the time but can ramp up as needed. It appears to run at the 25% level, as far as I can tell, most of the time. I doubt it cycles off at all. It is too bad The Villages didn't choose Carrier systems for the house with variable speed compressors. It is better for the system to run continuously at a lower compressor speed then to cycle on and off.

600 sq ft per ton is what is scheduled.