Decision Based On Size
You can come up with a general idea of what sized air conditioning unit you need based on home size. A house in a warm climate will need about a ton per 500 square feet of home space.
Therefore, for a 1,500 square foot house, you can plan on installing a 3 ton AC unit to your home.
A More Accurate Assessment
Although home size is a primary consideration when determining the proper size for an AC unit, there are other factors to consider such as specific climate, window configuration, insulation and building materials. For the most accurate evaluation, complete a "Manual J Load" calculation, which takes such specific information into account.
Generally, you can plan on installing a 3 ton AC unit in your home, but your builder or AC company should have you complete the "Manual J Load" assessment before a final decision is made.