What happens if your Air Conditioner is oversized?

Well that kind of depends on how oversized it is...if it's fairly close, generally the a/c unit will short cycle, meaning that it satisfies the space temperature too quickly. All of those starts and stops are hard on the machine, and generally you will have humidity issues in the space, because the a/c isn't running long enough to remove the humidity.

If it's way oversized, not going to work at all, or at least with not damaging the machine.