Upper panels: star-forming galaxies with in the Σ_SFR − Δ log(SSFR)−z plane. Dashed blue lines show the Δ log(SSFR) limit of −0.3 above which our sample of SFGs is complete in terms of distance to the MS. Black arrows show the upper limits for the size of unresolved sources. Middle panels: density distribution per Δ log(SSFR) bin (0.4 dex width) of SFGs in the Σ_SFR−Δ log(SSFR)−z plane. Contour levels are at the 10, 20, 30, 40, 50, 60, 70, 80, and 90th percentiles. The median size derived via the KM estimator is shown by the dark magenta circles; the error bars correspond to the 16th and 86th percentiles of the distribution. Lower panels: power law describing the Σ_SFR−Δ log(SSFR) relation (dotted black line). The slope and normalization are given in the lower right corner. The median size derived via the KM estimator is also shown by the dark magenta circles. For comparison, we present the compilation of SFGs from Genzel et al. (2010, black filled circles), Tacconi et al. (2013, black empty squares), Elbaz et al. (2018, red pentagons), and Lang et al. (in prep., orange triangles). The black arrow illustrates the factor to be considered when comparing Σ_SFR of galaxies in our sample with that reported by Genzel et al. (2010) and Tacconi et al. (2013) (see text for details).