Emil,
The AM radio antenna has a slightly easier job, since it does not have to measure the location of the photon, just detect that some sufficient part of its probability amplitude distribution overlapped with the antenna. On the other hand, another example of tiny photosites is film, where light detection is done by the intera tion of a photon with a small cluster of silver halide crystals, and AFAIK, those clusters are far smaller than the wavelength of visible light.
Anyway, one clear experimental fact is that the Nokia 808 with its 1.4 micron sensels and f/2.4 lens gives fairly sharp images at 100% pixel viewing, which shows that with a large enough aperture, resolution down to about 1.4 microns is possible ... and that would translate to 432MP in 35mm format.
P. S. It is interesting how many myths and dodgy arguments keep popping up to rationalize some current popular resolution level as the natural limit, like multiple arguments that going beyond Canon's current 22MP limit only makes sense in formats larger than 35mm, due to diffraction or whatever. I remember that back in the day of 6MP in EF-S and DX format, it was already claimed by some that "they've gone about as far as far as they can go" in that format, so that any future progress required "FULL FRAME". (Such people usually used all caps for those holy words.)