The light is produced by electrons combining with holes, so the size of the material doesn't come into it (unlike an antenna). I've personally pushed* a single rubidium atom about in a quantum computer and watched it move by emitted light.

* Ok, actually pressed buttons that manipulated the electric field that was trapping the atom and watched the result on a display - lot of physics going on behind the scenes.

When it comes to solid-state photon emissions, minimum emitted wavelength is not necessarily constrained to size of the structure, but rather the electrical bandgap that needs to be overcome. Electrons are much smaller than any photon, by about 2-3 orders of magnitude, yet it is their being trapped in quantum wells which creates light emissions with wavelengths many times their size.