In nearly all cases, an air conditioner that is larger than needed is a bad thing.  When an air conditioner is over-sized it will “short cycle”.  This means that the unit will turn on and off repeatedly.  This occurs because it produces so much cold air that it satisfies the thermostat too quickly.  It does not allow the unit to run long enough to remove enough moisture in the air, leaving the humidity level in your home much higher than is desirable, even to the point of causing dangerous mold and mildew problems.  This will also increase the number of hot and cold areas in your home since the air does not move enough to mix well throughout the entire home.  Secondly, the unit will be much less efficient if it is required to start up more often and will reduce the lifespan of the system.  In short… Bigger is NOT better when it comes to your HVAC system.  Have it sized right!