Question: I read the related answer to whether photons have size, and the answer seemed to be it depends. If a Photon, or a rather an E&M wave has a magnetic and electric field, should things be able to feel this? I know electrons do get 'sloshed' in these fields, but what is the range of influence of a photon? How much space do the waves physically occupy?

Answer: 
One has to have clear that the terminology "photon" describes an [elementary particle](http://en.wikipedia.org/wiki/Elementary_particle).

Elementary particles are described concisely in the framework of quantum mechanics. In this framework an elementary particle, the photon in this case, can have two behaviors. Either as a classical physics point particle with an x, y,z position , i.e. no extent in space, or as a **probability** wave , which means that a statistical accumulation of individual photons from the same starting conditions will display an [interference pattern](http://www.youtube.com/watch?v=GzbKb59my3U) as a classical wave would. In the wave manifestation the extent of the locus where the photon may be found is bounded by the [Heisenberg Uncertainty Principle](http://en.wikipedia.org/wiki/Heisenberg_uncertainty). This last tells us that the better we know the momentum of the photon, the less localized the locus of finding it is.In this sense the probable extent of a photon can be made as large as our knowledge of its momentum p=h/lamda , the planck constant over the wavelength. 

Now nature and the physics models we have developed to describe and predict its behavior [is continuous](http://motls.blogspot.com/2011/11/how-classical-fields-particles-emerge.html) between the photon framework of quantum mechanics and the classical electromagnetic waves of [maxwell's equations.](http://en.wikipedia.org/wiki/Maxwell%27s_equations) , the frequency of the photon is the frequency of the wave it will build up when present in large numbers. Electromagnetic waves can have the extent one designs and the limits are the limits of the ability to produce them and direct them. Radio waves are all over the place.
Perhaps a good way of thinking about it is that a photon is produced by some process: a free-space decay of an atom in free space, to a stimulated emission in a laser cavity, etc. These processes have a spread in energy(frequency) known as their bandwidth. Since frequency and time are conjugate variables in QM, the temporal length of the photon is constrained by the Fourier transform of the bandwidth, and since we know the propagation velocity of the photon, this defines it's length. The transverse dimension of the photon is given by the mode of the resonator that produced it (the dipole pattern of the atom, or the Hermite-Gauss mode of the laser cavity).

As an example, consider an atom in a laser cavity which has a mode waist of 1mm, and a bandwidth of 1 MHz (assume that the natural line width of the laser is much larger to keep it simple). The photon emitted from the cavity would be distributed along a volume which is transversely 1mm in diameter and longitudinally is c/1MHz ~ 300m long. As mentioned above, one typically views this as the probability envelope of a the photon so that over many identical realizations of the single photon event, one would find the above probability distribution in space or time.

From the standpoint of pure quantum optics, one formally considers a resonator in which the E-M field is a standing wave and has a lowest energy ground state. From here you find that the Hamiltonian is that of a harmonic oscillator and has single excitation (Fock) states which represent single photon. To think of photons in free space, one typically extends the dimensions of the resonator to infinity. This allows you to sleep better at night because it's rigorous, but I find viewing photons as described above more useful for understanding physics.