> What is the key reason to use photon size

Photon size wasn't used. Micron is an unofficial name for 1e-6 m, the lengths of 0.7e-6 m to 1e-3 m correspond to the wavelength of infrared radiation:

https://en.wikipedia.org/wiki/Infrared

So "those 10 micron photons" there mean "the photons of the radiation with the wavelength of 1e-6 m."