In Dallas, my 2 story 3200 sq ft has 8 tons that I would call "adequate". We entertain upstairs frequently (25-30 people). We have to start around noon and crank both 4 ton units down to 70 and stablize at that temp, then guests arrive by 8PM and then by midnight, we are lucky if we have kept it down to 78. So I guess it depends on the load.
Also I had rather have an inefficient overcapacity unit than a flashing wife :-)