Effective radius (R_e [arcsec]) plotted against the i-band magnitude (m_i) for the sources in our real sample of strongly lensed galaxies. The dashed line shows the best-fit line, given by Eq. (3). This magnitude–size relation (with scatter) is used to assign the effective radii corresponding to the drawn i-band magnitudes of the source galaxies in our simulated sample of strongly lensed galaxies.