  The formation of planets strongly depends on the total amount as well as on
the spatial distribution of solids in protoplanetary disks. Thanks to the
improvements in resolution and sensitivity provided by ALMA, measurements of
the surface density of mm-sized grains are now possible on large samples of
disks. Such measurements provide statistical constraints that can be used to
inform our understanding of the initial conditions of planet formation. We
analyze spatially resolved observations of 36 protoplanetary disks in the Lupus
star forming complex from our ALMA survey at 890 micron, aiming to determine
physical properties such as the dust surface density, the disk mass and size
and to provide a constraint on the temperature profile. We fit the observations
directly in the uv-plane using a two-layer disk model that computes the 890
micron emission by solving the energy balance at each disk radius. For 22 out
of 36 protoplanetary disks we derive robust estimates of their physical
properties. The sample covers stellar masses between ~0.1 and ~2 Solar masses,
and we find no trend between the average disk temperatures and the stellar
parameters. We find, instead, a correlation between the integrated sub-mm flux
(a proxy for the disk mass) and the exponential cut-off radii (a proxy of the
disk size) of the Lupus disks. Comparing these results with observations at
similar angular resolution of Taurus-Auriga/Ophiuchus disks found in literature
and scaling them to the same distance, we observe that the Lupus disks are
generally fainter and larger at a high level of statistical significance.
Considering the 1-2 Myr age difference between these regions, it is possible to
tentatively explain the offset in the disk mass/disk size relation with viscous
spreading, however with the current measurements other mechanisms cannot be
ruled out.
