  We present deep, near-infrared HST/WFC3 grism spectroscopy and imaging for a
sample of 14 galaxies at z~2 selected from a mass-complete photometric catalog
in the COSMOS field. By combining the grism observations with photometry in 30
bands, we derive accurate constraints on their redshifts, stellar masses, ages,
dust extinction and formation redshifts. We show that the slope and scatter of
the z~2 mass-size relation of quiescent galaxies is consistent with the local
relation, and confirm previous findings that the sizes for a given mass are
smaller by a factor of two to three. Finally, we show that the observed
evolution of the mass-size relation of quiescent galaxies between z=2 and 0 can
be explained by quenching of increasingly larger star-forming galaxies, at a
rate dictated by the increase in the number density of quiescent galaxies with
decreasing redshift. However, we find that the scatter in the mass-size
relation should increase in the quenching-driven scenario in contrast to what
is seen in the data. This suggests that merging is not needed to explain the
evolution of the median mass-size relation of massive galaxies, but may still
be required to tighten its scatter, and explain the size growth of individual
z=2 galaxies quiescent galaxies.
