Top panels: size–stellar mass relation for each of the simulation suites used in this work, where the stellar mass is measured within 20% of R₂₀₀. The median values and 1σ scatter of the stellar half-mass radius in each stellar mass bin are shown by the grey lines and shaded regions, respectively. Likewise, the median values and 1σ scatter of galaxy size defined as R₁ is shown for AURIGA (orange), HESTIA (blue), and NIHAO (green) by the lines and shaded regions, respectively. The sizes of individual galaxies are shown with dots (AURIGA), crosses (HESTIA), and squares (NIHAO), using the same colour criteria. Bottom panels: Normalised histograms of the distribution of galaxy sizes relative to the median ($\overline{R}$) for each simulation suite. In each case, the global σ of the relation was found by fitting a Gaussian distribution (solid line following the same colour of the respective histogram) and σ was defined as the standard deviation of the respective Gaussian, whose errors were computed via bootstrapping.