  Fits the infrared spectra from the nuclear regions of AGN can place
constraints on the dust properties, distribution, and geometry by comparison
with models. However, none of the currently available models fully describe the
observations of AGN currently available. Among the aspects least explored, here
we focus on the role of dust grain size. We offer the community a new spectral
energy distribution (SED) library, hereinafter [GoMar23] model, which is based
on the two-phase torus model developed before with the inclusion of the grain
size as a model parameter, parameterized by the maximum grain size Psize or
equivalently the mass-weighted average grain size < P >. We created 691,200
SEDs using the SKIRT code, where the maximum grain size can vary within the
range Psize = 0.01 - 10.0um ( < P >= 0.007 - 3.41um). We fit this new and
several existing libraries to a sample of 68 nearby and luminous AGNs with
Spitzer/IRS spectra dominated by AGN-heated dust. We find that the [GoMar23]
model can adequately reproduce up to 85-88% of the spectra. The dust grain size
parameter significantly improves the final fit in up to 90% of these spectra.
Statistical tests indicate that the grain size is the third most important
parameter in the fitting procedure (after the size and half opening angle of
the torus). The requirement of a foreground extinction by our model is lower
compared to purely clumpy models. We find that 41% of our sample requires that
the maximum dust grain size is as large as Psize =10um (< P >= 3.41um).
Nonetheless, we also remark that disk+wind and clumpy torus models are still
required to reproduce the spectra of a non-negligible fraction of objects,
suggesting the need for several dust geometries to explain the infrared
continuum of AGN. This work provides tentative evidence for dust grain growth
in the proximity of the AGN.
