Size and luminosity relations at 1.3 and 3 mm (circles). Left: R₆₈ vs. flux at 1.3 mm. Right: R₆₈ vs. flux at 3 mm. Fluxes in both panels have a correction for inclination that is valid for optically thick emission. The dashed lines show the linear regression results for Class II sources at the two wavelengths, derived in Tazzari et al. (2021a), which also include the correction for optically thick emission. The dotted lines corresponds to the relation for Class II considering a shift in the fluxes of a factor of 10.