The only way to accurately figure this is to know the location of the house, orientation, insulation levels, windows, sizing and orientation, and the size. You've only provided a very small amount of the info if you want to size the thing accruately. Ballpark sizing charts are often oversized, and the worst thing is an oversized a/c unit. Ideally, it runs constantly and just maintains the desired temp. This dehumidifies the air and there is little swing in temp. Obviously, no year has all days the same, so you size for the worst average (or a little less), and live with it being a slight bit warmer on that hottest day of the century.