EuclidJ_E FWHMs for sources identified as LBGs (blue dots) and UCDs (red stars) using SED fitting on HSC + VISTA + Euclid photometry. Also shown is the size-luminosity relation from HST imaging of a bright ground-selected sample Bowler17, and the prediction from the FLARES simulation (Roper et al. 2022). The dotted grey line is the median Euclid PSF FWHM. The grey shaded region shows the FWHMs of PSFs selected from JWST F444W imaging when they are observed in EuclidJ_E.