Most people think that a central A/C system should cool the house in a similar way that your car cools you; you turn it on and almost instantly you have ice cold air. Unfortunately, central A/C systems are not designed to cool the house instantly. If they did, then you would have a very cold, clammy and uncomfortable home.
A properly sized system should drop the temperature about 2-3 degrees per hour (depending on outside temperatures).
Instead of focusing on the thermostat setting, the more important consideration should be on overall comfort. The key to that is dehumidification. Your comfort level increases in direct proportion to the amount of humidity that is being removed. Oversizing results in short cycling and a cold, clammy home. Undersizing will definitely remove a lot of humidity but will be unable to maintain the desired temperature.
The only way to guarantee proper performance is to do a load calculation on your home. This will give you a system that strikes a proper balance between temperature drop and dehumidification. Many of the newer A/C systems have multiple stages of cooling along with communicating thermostats all designed to maximize your comfort.
