For example, if photons were the size of galaxies, would they still move at the speed of light?
The concept of "size" is ambiguous for particles. Photons can in principle have wavelength the size of a galaxy, and it still moves at the speed of light.
Other particles, except gluons, have mass so they travel slower than the speed of light. Gluons can't propagate freely, so don't have a speed.
How were you able to measure the size of a photon in the first place?
ALL light/EM radiation, by definition, travels at c in vacuum.
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