Left panel: fraction of mock sources retrieved by PyBDSF as unresolved in the plane showing the ratio of the number of unresolved sources per bin to the total number of unresolved sources in the entire parameter space. Right panel: cumulative size distribution of mock sources retrieved as unresolved, which represent 29% of the total number of sources injected in our MC simulations. Around 90% of them lie below θ_beam = 0.75 arcsec (blue line), hence we use this value as the upper limit for the size of unresolved SFGs in the VLA COSMOS 3 GHz map.