Does a photon have any measurable size as a "particle"? If we determine its exact location and take away all motion in an isolated static environment, how large is it? Is it sub-Planck in size? If there is no motion in this static environment, is the photon also sub-Planck in terms of time?
According to superstring theory, the physical size of any elementary particle (including photons) is that of one superstring, or the Planck length. As a wave though, we can think of the "size" of a photon as being that of its wavelength. Note however that the Heisenberg uncertainty principle means that trying to measure the amount of space that a photon occupies will proportionally increase the uncertainty in its momentum and energy. The Planck length would represent the smallest size that is physically possible to measure in theory (at one Planck energy).
In the standard model of particle physics which is continuously validated by experiments, the photon is a zero mass,spin one, point ,elementary particle in the table of particles that build up all that we know. Its energy is $h*ν$ where $ν$ is the frequency of the classical electromagnetic wave that it can build up in superposition with zillions of other photons.
If we determine its exact location and take away all motion in an isolated static environment, how large is it?
Zero mass means it is moving with speed c in all Lorentz frames as all zero mass particles, so this scenario can never happen.
Does a photon have any measurable size as a “particle”?
The size of the footprint of the photon will depend on the measuring instrument. The footprint will be bounded by the Heisenberg uncertainty principle .
Here one observes the footprints of single photons, in a double slit experiment.
The footprint is a dot within the measurement errors, and certainly bounded by the uncertainty principle, since h, the Planck constant, is very small, macroscopic measurements always obey it.
Thus , a photon, as described by current quantum mechanical theories is a point particle, and it demonstrates an effective size according to the boundary conditions of the measurement.