As we know EM waves have a wave length. Well, how do you measure them? I have the feeling there is a path from point A to point B and that path goes up and down at a regular rate and all photons travel along that wave-path and that they are no where else between A and B but on that path. But that view I have a feeling is incorrect. I think it's more probable that the photon occupies a probability space and that the line a photon is almost never here is a bit arbitrary and that is the length of the EM wave. Also, I want to double check. I looked at wiki and I saw that the size of a photon is not listed prominently. Is that because they are bosons and can occupy the same space? If they can occupy the same space, then you can't measure their size.
First, forget everything you think you know about photons. It is wrong. A photon is the quantized interaction of an electromagnetic wave. They do not have a size. They are not particles with mass that occupy space. As for an EM wave, the wavelength is talking about how far the wave travels during one oscillation of its electric and magnetic field vectors. There is nothing moving up and down. The little squiggly lines you see representing photons is utterly incorrect. The EM wave travels outwards like ripples on the surface of a pond, only in three dimensions, not two. If you were to measure the EM wave as it travels, you could graph the changing electric and magnetic fields on a graph. This is what you normally see when you look at anything that shows a photon or an EM wave. The distance between any crest or trough is the wavelength.
The simplest way to do it is by measuring interference patterns. Suppose that you have two identical sources of EM waves placed at a distance L between them. Put your detector at some equidistant point from the two detectors . It will start ringing like crazy! Then shift it aside , so that the distance from source 1 is r1 and from source 2 is r2. When it starts ringing again like before , the difference between r1,r2 will be equal to the wavelength.