Think the quantum mechanical double slits experiment. Quantum mechanical entities show the particle-wave dual nature in the experiment. It is said that an electron travels as a wave, but departs and arrives as a particle. A single photon does behave in the same way. Reading a book, it is written that a single photon makes a single white dot on the photographic plate in the double slits experiment. A single white spot? Oh, what a surprise! I have been thinking that the travelling photon must be described by the plane wave that extends infinitely in any direction that is perpendicular to the direction of propagation of the wave. Such an infinitely extending plane wave converges to a single spot? Then, if so, I would like to ask you a question.
How large do you think is the diameter of a single photon that is recorded on the photographic plate?
You say that a photon is a particle. If it is a particle, then it should have a measurable spatial extention.