2 tons keeps my 1500 sqft upstairs cool to 73F about 98F (above design temp)... and while it does have a lot of shading, there's no formal wall insulation and it has about 20 good sized windows. My 1700sqft downstairs wiht 9' ceiligns and about 20 taller windows will be getting a 2 ton unit when I replace it soon. It can almost get by with 1.5Tons, but throw in people and appliance loads and that pushes it to 2 tons. But each home is different.
Remember with properly sized systems you can't and do not want to use setbacks in the daytime in summer. IT will never catch up. A larger upstairs like your should have a properly sized AC running nearly constantly for 6-8 hours when it's over 90F, humid and sunny outside.