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.
At very low energy, the difference between photons and E&M waves becomes clearer: After every spin-flip in a hydrogen atom, "something" with E=6 µeV and spin=h/(2\*pi) is released. Is that "something" a photon? Nobody can detect a photon with such a low energy. But if you have a radio receiver at 1420 MHz, you get a clear signal. The electric component of the wave accelerates electrons in a wire called dipole antenna and you can measure a weak current. Where is a photon? Radio receivers do not detect photons!

If the atom releases a photon, something transforms it to a long wave packet. Where? Is the intermediate photon necessary? Perhaps the atom emits the wave packet. How? Nobody knows.

The optimal length of the dipole (0.1 m in this case) corresponds to the diameter of the wave packet: a shorter dipole provides a weaker signal, a longer dipole brings no additional gain.

The coherence length of this wave packet is about 60,000 m. You can calculate using the FWHM = 5 kHz, measured in astrophysics. This huge wave packet is a big contrast to a point-like photon.