The black histogram shows the observed distribution of the half light radii (in arcsec) of the z ~ 7 galaxy candidates brighter than J = 27.8 in the HUDF. At this magnitude limit the completeness of the J band image is higher than 80% even for the most extended objects. The galaxy with half light radius  ~0.25 arcsec is the HUDF-1095 one (template 4). The cyan solid line shows the uniform distribution in half light radius (template 7), while the dark pink and green solid lines indicate the two log-normal distributions in half light radius adopted by Ferguson et al. (2004) (template 8, M1) and by Oesch et al. (2010b, template 9, M2), respectively. The purple line indicates the distribution derived from Vanzella et al. (2009) (template 10, M3). All sizes are corrected for PSF. The dashed line shows the completeness of galaxy detection for different half-light radii derived by the performed simulations. This curve has been multiplied by a factor of 2 only for clarity.