[Bored chemist]

Size of photon makes sense also in QM: e.g. as width of wavepacket ( https://en.wikipedia.org/wiki/Wave_packet ), as uncertainty of quantum position operator, through adding terms in WKB approximation ( https://en.wikipedia.org/wiki/WKB_approximation ) etc.

So please elaborate what do you know about this basic question? (beside excuses)

Nice graph.
What are the units of the vertical axis?

Are you planning to state the "width" of a photon in volts per metre?

[jeffreyH]

What is the size and shape of the wind? We can feel the effects of the wind. The atmosphere obeys the mechanics of fluid dynamics but we can't normally 'see' it. So what is the size and shape of the wind?

[wolfekeeper]

I think there's a lot of confusion here. Photons are subatomic particles that, like electrons, have no discernible size in and of themselves. They have a quantum wavefunction that has extent though, with a wavelength that is the wavelength of the photon. As I understand it, Maxwell's equation is basically modelling the QM wave in the classical approximation when there are a large number of photons.

[Jarek Duda (OP)]

Regarding plane waves, shape of wind, there are EM waves also of huge sizes (e.g. https://en.wikipedia.org/wiki/Extremely_low_frequency ).
However, optical photons are very special type: e.g. produced by concrete deexciting atom, then distance/c later absorbed by another concrete atom.

There are observed delays in atomic processes, like ~21 attosecod delay in photoemission ( https://science.sciencemag.org/content/328/5986/1658 ) - EM radiation, being mainly response to electron dynamics, propagates ~6nm during this time.
So what exactly happens during such 21as of production of EM field of single optical photon?

[jeffreyH]

Attosecond chronoscopy  actually looks very interesting. Worth keeping an eye on for new developments.

https://journals.aps.org/rmp/abstract/10.1103/RevModPhys.87.765

Also of interest, new developments.

https://arxiv.org/abs/2009.04913

[Jarek Duda (OP)]

Indeed, it shows to be careful about the basic quantum idealization of wavefunction collapse being instant - they have some hidden dynamics we should search for ... like EM wave of single photon being produced by electron dynamics - such photon carries energy difference and angular momentum (orbital or spin) of electron.

~1000 articles citing 2010 Science "Delay in photoemission": https://scholar.google.pl/scholar?cites=15193546925951882986&as_sdt=2005&sciodt=0,5&hl=en
E.g. 2020 "Probing molecular environment through photoemission delays" https://www.nature.com/articles/s41567-020-0887-8

Attosecond chronoscopy has revealed small but measurable delays in photoionization, characterized by the ejection of an electron on absorption of a single photon. Ionization-delay measurements in atomic targets provide a wealth of information about the  timing  of  the  photoelectric  effect,  resonances,  electron  correlations  and  transport.

[yor_on]

I think this one is interesting.   https://aca.scitation.org/doi/10.1063/1.4997175

and the example given by Young Kindaichi purely fun.   https://physics.stackexchange.com/questions/597663/instantaneous-ejection-of-photoelectrons-indicative-of-particle-nature-of-light