  [Abridged]We present a study based on a sample of 62 early-type galaxies
(ETGs) at 0.9<z_spec<2 aimed at constraining their past star formation and mass
assembly histories. The sample is composed of normal ETGs having effective
radii comparable to the mean radius of local ones and of compact ETGs having
effective radii from two to six times smaller. We do not find evidence of a
dependence of the compactness of ETGs on their stellar mass. We find that the
stellar mass of normal ETGs formed at z_form<3 while the stellar content of
compact ETGs formed at 2<z_form<10 with a large fraction of them characterized
by z_form>5. Earlier stars formed at z_form>5 are assembled in compact and more
massive (M_*>10^11 M_sun) ETGs while stars later formed (z_form<3) or resulting
from subsequent episodes of star formation are assembled both in compact and
normal ETGs. Thus, the older the stellar population the higher the mass of the
hosting galaxy but not vice versa. This suggests that the epoch of formation
may play a role in the formation of massive ETGs rather than the mass itself.
The possible general scheme in which normal <z>~1.5 ETGs are descendants of
high-z compact spheroids enlarged through subsequent dry mergers is not
compatible with the current models which predict a number of dry mergers two
orders of magnitude lower than the one needed. Moreover, we do not find
evidence supporting a dependence of the compactness of galaxies on their
redshift of assembly. Finally, we propose a simple scheme of formation and
assembly of the stellar mass of ETGs based on dissipative gas-rich merger which
can qualitatively account for the co-existence of normal and compact ETGs
observed at <z>~1.5 in spite of the same stellar mass, the lack of normal ETGs
with high z_form and the absence of correlation between compactness, stellar
mass and formation redshift.
