A photon has size but probably not in the rigid sense that we think about with macroscopic objects. Although photons exhibit properties of particles (and we know the size and mass of subatomic particles such as protons etc.), they also exist as a wave. This is known as wave particle duality. A quick search has found this which helps explain the consequences of such a measurement.
https://www.quora.com/How-big-is-a-photon
The team tested two different photon shapes – one rising in brightness, the other decaying. Hundreds of millions of measurements made over 1500 hours showed that the overall probability that a single Rubidium atom would absorb a single photon of either type was just over 4%. However, when the team looked at the process on nanoscale timeframes, they saw that the probability of absorption at each moment depends on the photon's shape.
http://phys.org/news/2016-12-interaction-atom-photon-quantum-devices.html
Point-like particles are mathematical abstractions with zero size. But even zero-size particles have an extended effect, due to the effect of the field surrounding them.
https://www.fnal.gov/pub/today/archive/archive_2013/today13-02-15_NutshellReadMore.html
In CPH theory photons are combination of positive and negative virtual photons. Photon is a very weak electric dipole that is consistent with the experience. In addition, this property of photon (very weak electric dipole) can describe the absorption and emission energy by charged particles. So, a photon has size and shape.
https://www.researchgate.net/publication/303988070_Generalization_of_the_Dirac's_Equation_and_Sea?ev=prf_pub
Article Generalization of the Dirac's Equation and Sea
To be a Photon or a Light beam - that is the question :
In the light of our present understanding, energy manifest in two and only two forms:
A) as localized ponderable matter composed of charges, nullified or conjugated or alone, -that moves at sub-luminal speeds, or up close to light speed as the elusive neutrinos move.
B) as photons that move at the speed of light, almost as at which the elusive neutrinos move at.
However, photons are mainly attributed to electromagnetic waves that can have any frequency, from almost zero to an unattainable infinity. Still low frequency photons are as common as their high frequency cousins, that constitute such phenomenon as optical light, x-rays, gamma rays and beyond - while low frequency photons are indeed responsible for the transmission of energy via wires, which act as wave guides for the same photons - who manifestly exhibit as an instantaneous monopole at each instant, and indeed deflect both magnetometers and electrometers alike that are positioned in their path, - to the very extent of being highly regulated by government agencies, based on their intensity up to their ionization capacity around most of our civilized world.
Thus we must argue, that a single photon does not exist alone in space, but only as an interaction when subjected to ponderable matter capable of accepting and emitting energy via their specific structure - weather a simple 1D string or a 2D surface, or a 3D volume - while all with their eccentric frequency selectivity.
So the old saying is true, that there is no sound without a listener.
This brings in the concept of topology, where EM radiation known as photons are simple - but can be engaged and knotted into a pair of electron/positron - and in fact any particle/antiparticle entity - as everything ultimately stems from light - as well as all particles will ultimately annihilate into light on meeting their antiparticle.
To summarize, this is very good news, as it exhibits the simplicity of photon-matter interaction, representing the same elementary substance - but only differing in topology, bringing Maxwell back into the picture along with the discreet reality of atomic and subatomic realty - as who could have imagined that the radius of the Proton is exactly 4 times its Compton radius, after decades of painstaking measurements, as my equation for the Proton Charge Radius, r = 4·ħ / c·m = 0.841235642... (fm), has now been confirmed to 0.1% precision, and is exactly 4 times its Compton radius !
Dear Gudlaugur
In generally, I agree with you and thank you for your considerable comment. What is your opinion about "Mapping the interaction of a single atom with a single photon may inform design of quantum devices" ? For detail see:
https://www.researchgate.net/publication/311962908_New_reasonable_evidence_of_CPH_Theory%27s_predictions?ev=prf_pub
Article New reasonable evidence of CPH Theory's predictions
Dear Hijaz
I agree with you that “There are still many puzzling aspects of the nature of light”.
http://iopscience.iop.org/article/10.1088/1367-2630/9/11/414/pdf
But disagree about rest mass of the photon.
There are many articles that show, the photon has upper limit mass and electric charge, which are consistent with experimental observations.
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.111.021801
http://www.sciencedirect.com/science/article/pii/037026938890723X
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