Source median angular size vs. 1.4 GHz total flux density, as estimated in some of the deepest radio samples available so far. Different colors/symbols correspond to different observing frequencies: 330 MHz (gray filled diamonds – Owen et al. 2009); 1.4 GHz (black filled triangles – Richards 2000; Bondi et al. 2003, 2008; Muxlow et al. 2005; Prandoni et al. 2018); 3 GHz (red empty squares – Bondi et al. 2018; Cotton et al. 2018); 5.5 GHz (blue stars – Prandoni et al. 2006; Guidetti et al. 2017); 10 GHz (brown asteriscs – Murphy et al. 2017). Also shown are the median sizes measured in our raw and final catalogs (150 MHz), combined together (filled black-bordered magenta squares and golden circles). We note that Guidetti et al. (2017) gives different median sizes for the AGN and star-forming galaxy subpopulations. The latter population is indicated as a circled blue star in the figure. All flux densities have been converted to 1.4 GHz, assuming a spectral index α = − 0.7. Various median size – flux density relations are shown for comparison: the ones proposed by Bonaldi et al. (2019) and Windhorst et al. (1990) (dot-dashed and long-dashed lines respectively), and some revised versions of the latter. The short dashed lines show the relations obtained by rescaling the Windhorst et al. (1990) relation by 1.5 × and 2 × (i.e., assuming k = 3 and k = 4 in Eq. (7)); the dotted line assumes a smooth transition between m = 0.3 and m = 0.5 going from mJy to sub-mJy flux densities,a s described by Eq. (8); the solid line assumes a value of k varying with flux density according to Eq. (9) (see text for more details).