The best way to size a heating or cooling system is by performing a heatload calculation to determine the area’s heating and/or cooling requirements. But, a heatload calculation is an approximate, it is a necessary approximate, but an approximate just the same. No one knows more about the heatload on your house than you, if you have lived there awhile. You should know how much time the air conditioner runs on hot days. If it runs all of the time, then I would say there is no spare capacity. If it only runs 45 minutes out of every hour, I would say you have another 25 percent capacity to spare. Compare the extra capacity of the air conditioner to the size of the addition in relation to the size of your house. For instance, if the addition, windows included, is only ten percent more space, then I would say the existing unit is big enough.