As long the capacitors have a width much larger than the distance between them, the energy stored is (if I recall correctly) linear. this is because the electric field is a constant vector field between the plates.

Once that is not true, you will transition to just separating two charged objects. Use Coulombs law to figure out the force. (Work energy is integral of force over distance).

The practical limit is not having electrons flying off due to the photoelectric effect or discharging due to conductive material in the environment.

So do it in a vacuum, in complete darkness.

Much shorter distances can be useful: https://en.wikipedia.org/wiki/Microphone#Condenser_microphon...