Spatial resolution in R_S for an angular resolution of 0.1 mas versus flux density at 7 mm. In blue, we plot the sources in our sample, in red, the well-known radio galaxies previously studied on these scales (Boccardi et al. 2021). For Cyg A, NGC 1052, and M 87, we assume the average 7 mm flux densities from Boccardi et al. (2016b), Baczko et al. (2019), and Walker et al. (2018), respectively. For NGC 6251, we refer to Cheng et al. (2020). For Cen A, no 7 mm VLBI flux density is reported in the literature and we estimated it from the 1 cm one (Müller et al. 2011), assuming α = −0.5.