Results of our simulations for z ~ 7 galaxies with Sersic profiles of index n = 4 in the HUDF field. The plot shows the comparison between the half-light radius (in arcsec) as measured by SExtractor as a function of the input one. The upper and lower panel refer to different total magnitudes, as reported in the legend. The red line shows the identity relation. The blue points and errorbar shows the average value and the relevant rms of the output half-light radius. At small sizes the output half light radius is typically larger than the input one due to the convolution with the instrumental PSF carried out during the simulations.