>The question being, we're able to describe the physical wavelength in nanometers of these waves that apparently aren't oscillating in space so much as they oscillate between electric and magnetic fields. ...how do you assign a unit of length to that?

The length is assigned to the distance it takes for the photon energy's wave to travel from one amplitude to the other and back. What that means physically *is* the phaenomena it expresses itself in, in the relevant experiments. If I was to speculate about how it's expressed as a logical principle of physical interactions, I'd say the wave is the photon's potential to bend or change direction, especially in contact with other matter (and the energy that constitutes said matter) as it travels through spacetime. But I'd also acknowledge that there's probably a reason why, even if my logic is correct, physicists would never oversimplify it like that.