How do I size a central Air unit?

We live in South Central MS, so our weather might be close to yours. We built our house 9 years ago. We have lots of big double pane windows. We have an open floor plan with the living room ceiling 2' higher. We have
6" walls with double insulation in the attic. Our a/c unit is 4 ton 10 seer. It does a good job keeping the house cool, but it will run non stop when the temps reach 100* or above. I hope this helps you.
Oh, our sq. footage is 2352.

You may want to look at getting triple pane windows. It will cost more but will pay for it self versus double pane being that you are in the deep South.

It seems like you are going to have flat ceilings in the bedrooms and baths.
If you go with the spray foam insulate have them spray the ceilings instead of the rafters. That way you arent heating or cooling the attic space.

Plenty of ventilation in those areas to draw the heat out.

Cooling size generally figured at 650 square foot per ton. But having vaulted ceilings might change that.

For heat I would do a electric furnace with A/C. Reason being why have 2 units running year round. Save the outside A/C unit some usage when it is not needed.