Here is my answer. Add any clarifications you might have.. Since the photon is a wave, the answer is similar to the answer to the question, "What is the size of a wave?" The size of a photon depends on how precisely you know the photon's momentum, or equivalently, by deBroglie's relations, how precisely you know the photon's spatial frequency (= f = 1 / λ). This is true of all waveforms/signals, and is summarized by the Δx × Δf = constant theorem. What if a photon imparts it's energy to an electron in an atom? The photon will then had to of transferred all of its energy to the electron wave, Standard quantum mechanics states that the photon wave function collapses, but gives no explanation as to how this happens. Standard quantum mechanics regards the waveform as giving only probabilistic information, and that modeling is content with only knowing the probability that a collapse will happen, not how it happens.