Most salesmen will give you a one ton/500 sqft value. And that may, or may not, work for your application; "your mileage may vary."
In Southern California, even a 3-ton A/C for 2000 sqft might be a bit much, since the air is so dry. But, you have to put up with relatively long run cycles, compared to running a 4-ton A/C in the same house. For power efficiency, you want longer run cycles, but for faster cooling or more drying, you might want the higher tonnage.
TTFN
FAQ731-376