The traditional measurement of the size of galaxies, the effective radius, is a relic of the epoch when shallow imaging was unable to capture the full extension of astronomical sources. However, current deep imaging surveys have revolutionised our view of galaxies and the characteristics of their faint outskirts, allowing us to regard critically our own conventions. From this perspective, it is time to move from a size definition based on the light concentration of galaxies, the effective radius, to a definition that intuitively captures the concept of the size of galaxies, such as its edge or boundary. In this talk, a physically motivated definition for the size of a galaxy based on the gas density threshold value for star formation in galaxies is studied. Remarkably, the new size definition not only captures what the human visual system identifies as the edge of a galaxy, but also dramatically decreases the scatter in the stellar mass–size relation by more than a factor of two. Compared to other size measures, the new parameter is unique in that it also unifies galaxies spanning five orders of magnitude in stellar mass on a single mass–size relationship. To place this discovery in the context of galaxy formation and evolution, its application is discussed on the understanding of the origin of discs and the nature of ultra-diffuse galaxies.