Rule of thumb can be quite expensive! You need to do a manual-j heat load analysis. If you do a search, you can probably find a free calculator, or have the vendor do it for you. It depends on the insulation, windows, orientation, and local ambient conditions. AL can be quite hot and humid. To remove the humidity means if the a/c is too large, it won't run long enough to pull the moisture out of the air and you'll be cold and clammy. You're better off with one slightly small verses one too big as you'll be more comfortable at a higher temp if the humidity is low, verses cold, but clammy.