No, you don’t want your air conditioner to be too large. Air conditioners control the comfort level in your home by cooling the air and by removing humidity. An over-sized air conditioner can cool your home faster, but it will use more energy and will not remove humidity adequately. In addition, your existing duct work may not be able to support the airflow of a larger system.
A unit that is too big for your home will have short run cycles. It can take only a short time to cool the air, but the unit shuts off before enough air blows across the indoor coil where moisture condenses into water and drains from your system. Too much moisture left in the air can lead to mold and mildew problems. These short run cycles also mean your system starts and stops more often which uses more energy and causes a lot of wear and tear. An air conditioners operate more efficiently during long run cycles.
The same holds true with heating systems. An over-sized furnace can warm the house quicker, but it uses more fuel and causes greater temperature swings in the home.