Central Air means he has air conditioning.....basically it all depends on how much you need to bring down the ambient temperature, For example, if your room temperature is 72, and your pumps, and lights and such bring total water temp to 81, you might only need to bring the water down a few degrees, and could go with a more under powered. If room temperature is higher like 80, it would increase the work on the chiller, so you'd need one more powerful.
It's not really a "how big " it's more "how much" with chillers.