How should disc brake rotor size scale with wheel size? I was wondering whether 20" wheels can get away with smaller rotors than 28" wheels, and if there is a correlation, if it is linear. If it was linear, then 140mm discs on a 20" wheel would be equivalent to 200mm discs on a 28" wheel (and it would be unnecessary to mount anything bigger for pretty much any application).

At a given forward speed, the circular speed of the rotor scales up linearly with decreasing wheel size, which on smaller wheels should help with the cooling and should provide higher friction power and thus higher braking power.

But I can't find any source confirming this, so I'm asking here.
To achieve the same braking power, the rotor/wheel diameter ratio should be constant. (Assuming the caliper is the same, and assuming cooling is sufficient.)

If the rotor/wheel ratio is constant the leverage between the force applied by the road surface and the rotor is constant. 

Also, because circumference scales linearly with diameter, the speed of the rotor surface at the caliper, and hence friction force, will be the same. 

Re heat dissipation, rotation of the disc will have little effect compared to  forward motion through the air, which will be much more effective at providing cooling air flow.

I would think that for most bikes that are not being used for sustained braking on descents an appropriately sized rotor and brake caliper has sufficient heat dissipation. I’m perfectly happy to be informed otherwise though.