  A key parameter governing the secular evolution of protoplanetary disks is
their outer radius. In this paper, the feedback of realistic dust grain size
distributions onto the gas emission is investigated. Models predict that the
difference of dust and gas extents as traced by CO is primarily caused by
differences in the optical depth of lines vs continuum. The main effect of
radial drift is the sharp decrease in the intensity profile at the outer edge.
The gas radial extent can easily range within a factor of 2 for models with
different turbulence. A combination of grain growth and vertical settling leads
to thermal de-coupling between gas and dust at intermediate scale-heights. A
proper treatment of the gas thermal structure within dust gaps will be
fundamental to disentangle surface density gaps from gas temperature gaps.
