I used a 203 BB7 on the rear of my 24" (and yes, I can set up a BB7 very well - they are the only brakes I ever use on the front), for modulation it was good, for bite & hold however it was in no way as good as a rim brake set-up.
So considering the smaller wheel I'd say a 185 rotor would be acceptable, but don't expect to be blown away with 'power'. I can just see a 160 rotor on a cable opperated disc brake on the rear not being enough..