now i do have a 1200 sq ft commercial metal building on a slab, quite a few windows, normal insulation in walls & ceiling, sheetrock walls, carpet floors. Hvac guy figured 3 ton unit with his load calculation last year. Seemed kind of large at the time but now summer has hit, its not one big too large. In fact any smaller and it would have struggled to keep it at 77 degrees.
i gave two people my room measurments , window sizes , and ceiling heights , and the both came up with one saying a 3-ton will be to large and the 2-ton will be alright . and the other one said visa - vesa.
The load calc on this site would be money well spent. You might need a 2.5 ton. Noone knows without performing a load calc. The "rule of dumb(thumb)" is 1 ton per 500 sq ft, which can oversize by today's standards as houses are built tighter.
If everything was always done "by the book"....the book would never change.