Generally, the bigger the better. I have a 12k gallon pool with a cell rated for 40k gallons. A larger cell means you can turn down the "output" which is actually more of a timer type mechanism. The end result is you have the same chlorine output with less runtime (if you get a bigger cell vs getting a smaller one). It saves electricity (although swcg don't use a lot anyway) and more importantly will make your cell last longer before it has to be replaced. As you may or may not have noticed, they're fairly expensive and they wear out over time.

I wouldn't get a huge commercial pool sized unit...


This is covered in the SWCG F.A.Q. page here: http://www.poolsolutions.com/gd/swim...ators-faq.html It's definitely worth the time to read it if you haven't already.