Current quantum theory consists of laying a quantum framework over a classical space-time. All the forces except gravity are part of the quantum framework. Gravity is accounted for by the classical behavior of space-time. As I understand it (from reading the wiki and other sources), when you work with things on the Planck scale, the assumption that space-time is classical breaks down, and you have to deal with a quantum theory of gravity. Until we have a tested theory of quantum gravity, we won't be able to model what happens at the Planck scales. It might well be that something that small can exist, but it would be described by something beyond our current models.

As to particle size limits, our current models predict that fundamental particles interact as infinitely small points. However, in between interactions, they are described by quantum wavefunctions which are spread out over some small region of space (called the Compton wavelength). So long as the wavefunctions are spread out over regions of space larger than the Planck length, current quantum field theory can describe them. If the particles were to have Compton wavelengths smaller than the Planck length, a new theory would be needed.

I believe this also applies to photons, where the Compton wavelength is simply the wavelength of the photon. This means in practice that at small enough wavelengths (high enough frequencies), current quantum field theory probably doesn't do a great job of describing photons.

There is one issue with this that I'm not 100% sure on. You can always change the frequency of light relative to yourself by simply accelerating which introduces a Doppler shift. You could imagine a photon had high enough energy that it was just on the verge of the Planck scale. If you accelerated towards it, you would see its wavelength decrease relative to you and drop below the Planck scale, but someone who hadn't accelerated would still see the photon above the Planck scale. I'm not sure how to interpret that...