According to Quantum Mechanics, a photon, like any other point particle, has no size at all. It is a point, if treated as a particle. If it is treated as a wave, it has wavelength and amplitude instead, amplitude being the "size".
I've always though of particles in the following sense:
If you do NOT measure/decohere a particle in some way, it exists ONLY as a probability wave--there is no "actual" number for each of its unknown quantity, just a probability of what it will be. There are no "hidden variables" that we...