  [abridged] The mass-size relation of early-type galaxies (ETGs) has been
largely studied in the last years to probe the mass assembly of the most
massive objects in the Universe. In this paper, we focus on the mass-size
relation of quiescent massive ETGs (Mstar/Msol > 3*10^10) living in massive
clusters (M200 ~ 10^14 Mstar) at 0.8< z <1.5, as compared to those living in
the field at the same epoch. Our sample contains ~ 400 ETGs in clusters and the
same number in the field. Therefore, our sample is approximately an order of
magnitude larger than previous studies in the same redshift range for galaxy
clusters. We find that ETGs living in clusters are between ~30-50% larger than
galaxies with the same stellar mass residing in the field. We parametrize the
size using the mass-normalized size, gamma=Re/Mstar^0.57. The gamma
distributions in both environments peak at the same position but the
distributions in clusters are more skewed towards larger sizes. Since this size
difference is not observed in the local Universe, the size evolution at fixed
stellar mass from z~1.5 of cluster galaxies is less steep ((1+z)-0.53pm0.04)
than the evolution of field galaxies ((1+z)-0.92pm0.04).
  The size difference seems to be essentially driven by the galaxies residing
in the clusters cores (R<0.5*R200). If part of the size evolution is due to
mergers, the difference we see between cluster and field galaxies could be due
to higher merger rates in clusters at higher redshift, probably during the
formation phase of the clusters when velocity dispersions are lower. We cannot
exclude however that the difference is driven by newly quenched galaxies which
are quenched more efficiently in clusters. The implications of these results
for the hierarchical growth of ETGs will be discussed in a companion paper.
