Upper panels: maximum radius of the Lyα haloes detected using the CoG method as a function of the absolute UV magnitude of their host galaxy. The grey contours correspond to the predicted virial radius/UV magnitude relation predicted by a semi-analytic model (Garel et al. 2015, contours at 10^-2, 10^-3, 10^-4, 10^-5 percent of the total number of modelled galaxies). The dashed black line corresponds to a polynomial fit of the distribution of the simulated galaxies. Each panel corresponds to a different redshift bin. The plot aims to show what cold CGM scale we probe with Lyman alpha emission. Lower panels: ratio of the predicted median virial radius at a given UV magnitude over the measured CoG radius of individual objects, plotted as a function of absolute UV magnitude in different redshift bins. The median values are indicated by the dashed purple lines.