To figure out the right size a/c unit...try this method
I saw this on the internet. Take the square footage of your home and multiply it by 25, then divide this amount by 12,000. This will tell you the size (ton) you will need for your central a/c. Example....2000 sq ft. home multiply by 25 equals 50,000 divided by 12,000 equals 4.16. You would need a 4 ton unit.