  We discuss how the effective radius Phi(Re) function (ERF) recently worked
out by Bernardi et al. (2009) represents a new testbed to improve the current
understanding of Semi-analytic Models of Galaxy formation. In particular, we
here show that a detailed hierarchical model of structure formation can broadly
reproduce the correct peak in the size distribution of local early-type
galaxies, although it significantly overpredicts the number of very compact and
very large galaxies. This in turn is reflected in the predicted size-mass
relation, much flatter than the observed one, due to too large (~3 kpc)
low-mass galaxies (<10^11 \msun), and to a non-negligible fraction of compact
(< 0.5-1 kpc) and massive galaxies (> 10^11 \msun). We also find that the
latter discrepancy is smaller than previously claimed, and limited to only
ultracompact (Re < 0.5 kpc) galaxies when considering elliptical-dominated
samples. We explore several causes behind these effects. We conclude that the
former problem might be linked to the initial conditions, given that large and
low-mass galaxies are present at all epochs in the model. The survival of
compact and massive galaxies might instead be linked to their very old ages and
peculiar merger histories. Overall, knowledge of the galactic stellar mass {\em
and} size distributions allows a better understanding of where and how to
improve models.
