Answer
A change in the distribution of electrons in the electron cloud.
The size of the electron cloud.
Work Step by Step
Dispersion forces are caused by the electron cloud of a molecule or monatomic atom being attracted to the nucleus of a nearby atom or molecule causing a change in the distribution of electrons. This creates temporary dipoles where there are more electrons and contributes to attraction between species.
The strength depends on how easily the electron cloud of one molecule can be distorted by a positive charge. This is a property know as polarizability and it's greater when there are more electrons/a larger electron cloud, as there is less attraction to the nucleus and they can be pulled more easily by charge.
