The spatial extent of an electromagnetic wave is determined by its wavelength. The wavelength represents the distance between two consecutive points of the wave that are in phase, meaning they have the same position in their respective cycles.
In the context of light and photons, the concept of amplitude refers to the wave's intensity or brightness rather than the probability of a photon's location. The amplitude of an electromagnetic wave represents the maximum displacement of the electric and magnetic fields comprising the wave.
However, when considering the particle-like behavior of light as described by quantum mechanics, the wave-particle duality comes into play. In this framework, the probability distribution for finding a photon is described by the wavefunction, which is related to the amplitude of the associated wave. The wavefunction provides information about the probability of detecting a photon at different locations.
It's important to note that the wave nature of light, including its spatial extent and probability distribution, is described by wavefunctions, while the particle nature is associated with discrete packets of energy called photons. The relationship between the wave and particle aspects of light is a fundamental principle of quantum mechanics known as wave-particle duality.