Top: radio spectral index between 325 MHz and 3 GHz as a function of starburstiness or the distance from the main sequence (parameterised as SFR/SFR_MS). For reference, the horizontal dashed line indicates the median spectral index value of (see Table 3). Bottom: 3 GHz radio size (radius calculated as θ_maj(FWHM)/2) from Paper II as a function of the distance above the main sequence. The blue vertical dashed line indicates the median value of SFR/SFR_MS = 5.2 derived for the plotted sample, while the horizontal line indicates the survival analysis-based median radius of 2.03 kpc for the plotted sample. The thin, red vertical dashed line shows the upper boundary limit of the main sequence, i.e. SFR/SFR_MS = 3.