I use a 15 gallon DeWalt that makes 200 PSI. I set the regulator to about 60 PSI for the clear wax and 50 for the hard wax and it hardly ever comes on. With low psi settings like that you could probably get away with one of those little 6 gallon jobs. As you probably already know however, the smaller the compressor, the more it will come on and you might have to wait for it to catch up.
I like my 15 gallon a lot. I get lots of use from it with air tools, waxoyl, beach toys, sprayin' out the garage, etc. If you want the smallest possible though, I'd call RN and ask 'em what they'd recommend. My 2 cents.