*(Its, or associated. That is somehow the question).
I can think of, at least in principle, a perfectly reflecting optical cavity with dimension comparable to the wavelength of the electromagnetic wave associated to a photon. Say, a kind of internally reflecting sphere with diameter of a wavelength or smaller (or a comparable cube etc for that is matter).
Would be possible for that photon to exist inside that object?
How this relates to the "size of a photon", where now size stands for the spread of the probability to find it (sort of "De Broglie wave for a photon") and what would change in this respect if the photon won't be confined?
I ended up to this while thinking about the meaning of the various jiggling arrows we draw when depicting photons arriving at a place, motivated by the common widespread feeling of getting lost when asked what it really means. In other words by thinking of a wave that, tough interacting as a point particle, must conveys information about an oscillating em field. It seems it needs at least a wavelength (in that medium) to do so. I hope that the question above is more clear than the motivation that drove me to write it.
Best Answer
Confining a photon to a region smaller than its wavelength is not only conceivable in principle, it is current experimentally achievable reality.
In particular in plasmonics, people build cavities that confine the electromagnetic field to such tiny regions for a time of multiple oscillation periods. Here is a review from 2010: "Review on subwavelength confinement of light with plasmonics" Journal of Modern Optics 57, 1479(2010) and a lot more work in this direction has been done recently. Another nice example is are "picocavities" such as in the more recent paper Science 354, 726-729 (2016).
So with regards to the question
we can give a clear "yes!" answer.
With regards to the more conceptual question on the size of a photon, the confined photon can be described as a superposition of various field degrees of freedom. So there is no conceptual problem here. Whether it is a single photon or not depends on which quantum state precisely you excite in the system.
In the case of plasmonics, specifically, one actually does not have a purely photonic excitation, but an excitation of the electrons in the confining cavity material is mixed in. This mixed excitation is called a plasmon.
Since there have been some comments on whether a plasmonic excitation qualifies as a photonic excitation or not, I decided to include a second example that avoids this discussion altogether: "Experimental realization of deep-subwavelength confinement in dielectric optical resonators" Hu et al., Science Advances 4, 2355 (2018). This paper (as one can see already from the title) experimentally realizes subwavelength confinement of the electromagnetic field in an all-dielectric photonic crystal cavity made of silicon. So this is purely based on refractive scattering of photons inside the resonantor.
This example unambiguously qualifies as "confining photons to less than their wavelength" without the definition discussion in the case of plasmonic resonators.