Radio SED of a sample of flat-spectrum sources, produced by selecting all sources having sizes less than 1 kpc. Red line shows the broken power-law model. We find that selecting sources with sizes less than 1 kpc produces a sample of flat-spectrum sources, described by a spectral index of α = 0.41 ± 0.07 and a broken power-law spectral index of α₁ = 0.1 ± 0.1 (α₂ = 0.55 ± 0.09) below (above) a break frequency of ν_b = (2.7 ± 0.5) GHz.