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tokens: larger than the PSF FWHM (orange dashed line). The 3D half-light radii agree with the two observational methods for the bright galaxies. The sizes of faint galaxies show a steeper decline than the observational data, indicating potential systematic errors in observations below the instrumental resolution limits. We examine how well the measured sizes using the three different methods recover the true 3D half-stellar mass radii. Fig. 10 illustrates the ratio of the F115W (rest-UV) sizes to the 3D half-stellar mass radii as a function of stellar mass, with the F115W sizes obtained by the three methods. We have combined sampled from simba -25 and simba -50. Solid green lines represent the median of the ratios, and the shaded regions show 1σ spread in the size ratios. For the growth curve method and 3D half-light radii, we also show results using the no-dust sizes with magenta lines. Figure 10. Similar to Fig. 9 , but showing comparisons of the measured F115W sizes (rest-UV) to the galaxy 3D half-stellar mass radii as a function of stellar mass. Shaded regions represent 1σ spread in the size ratios. The 3D half-light radii traces the half-mass radii very well at z = 6. Due to the finite pixel size and smoothing by the PSF, size measurements in real observations bias the ‘true’ sizes, which we find to be a mass-dependent effect. Focusing first on the right-hand panel of Fig. 10 , the no-dust 3D half-light sizes are smaller than the 3D half-stellar mass radii, but the ratios are roughly constant. This indicates that although more concentrated than mass, light traces mass well in these simulated z = 6 galaxies, so these galaxies can only have weak age gradients. When including dust extinction, the sizes could be a factor of 1.5–3 larger than sizes without dust for galaxies with M * ≳ 10 9 M ⊙ , and also overestimate the half-mass radii by up to a factor of ∼1.5. This suggests that the centres of the galaxies are more dust-attenuated. Indeed, we will show in Section 3.4.3 that the centres of these galaxies are younger but with more dust extinction. Results obtained with the two observational methods are generally similar, as shown in the left-hand and middle panels. The sizes of galaxies with M * 10 8 M ⊙ are overestimated by factors of 1.5–2, owing to the finite resolution of the mock images and smoothing of the PSF. Overall, simba produces galaxy sizes that are in good agreement with current HST observations at z = 6. We also find that the observed sizes are a biased tracer of the true half-stellar mass radii. Moving forward, we will explore predictions for JWST observations. 3.4.2 Rest-optical versus rest-UV sizes Since future JWST observations will be able to explore the z ∼ 6 galaxies in the rest-frame optical wavelengths, we explore how the rest-frame optical sizes compare with the rest-frame UV ones. Fig. 11 shows comparisons between the F444W (rest-optical) sizes and the F115W (rest-UV) sizes, measured using the growth curve method, the Sérsic