[jamest (OP)]

Sent in by Donald

[Janus]

That's a bit of a complex question, as it doesn't have a simple answer.  In one way, a photon is considered a dimensionless point particle, but in many practical applications, it is convenient to consider it "size" as being equal to its wavelength, which is dependent on its frequency.  So,in a sense, a photon's size can be from zero to whatever. 

[Eternal Student]

I'm not sure what happened with the datestamps and software here.   This looks like an old question and it was already answered in an old podcast.   However, it's displaying as if some of these posts appeared "yesterday".    Today is 07/06/2025 in the notation  (dd/mm/yyyy).   Maybe just a temporary glitch?

[evan_au]

...and the wavelength varies with the observer's speed, relative to the source (eg a laser), due to Doppler shift - and relativistic effects if you are traveling very fast, or in a deep gravitational well.

So the "size of a photon" (if it is considered to be non-zero) is not a property of the photon itself, but also of the observer.