What size ac for a 1500 square foot house?

There is no "one size". The number of Windows, orientation of the house, insulation, climate, and a number of other factors influence the sizing of the AC unit. If the unit is improperly sized, the energy consumption will be high and the life of the unit will be shortened.

The only way to determine the correct size is to do a Manual J calculation, or use software that implements that method.