Surfactants, in particular nonionic surfactants, form phase-separated domains when competitively displacing protein films from a fluid interface. The present article seeks to quantify the size, shape, and number of the growing domains as the displacement progresses. The two proteins used in this study, beta-casein and beta-lactoglobulin, were separately displaced by the nonionic surfactant Tween 20. In the cases of both proteins, the mean surfactant domain area was found to increase exponentially as a function of the fraction (area) of the interface containing surfactant. Similarly, the number of domains per unit area was found to decrease rapidly for both proteins as a function of the surfactant area fraction. While computer simulations of domain growth could not closely mimic either of these traits, they suggested that a likely explanation for this is that the rate of domain coalescence is controlled by the strength of the protein film. The mechanically stronger (more elastic) beta-lactoglobulin films were better able to resist the coalescence than the mechanically weaker (more viscous) beta-casein films. No simple relationship was found between the domain shape and the other measured properties of the film, except for the case of aged beta-casein films, where the domain shapes became markedly less circular.