I have a pair of 16 3/8 on the front (almost 18in) and a 24in on the rear, and I can say that combo is good enough for comfort but it understeers on the little Brompton front wheels. I blame the Marathon tyres on them, and I'm looking at putting fatter tyres on 18s (can't get them for the 16 3/8s).

I don't know how people use big wheels on the front. My turning circle will suffer with Big Apples on 18s and I can only just get it through a doorway.