  We present a new approach aimed at constraining the typical size and optical
properties of carbon dust grains in Circumstellar envelopes (CSEs) of
carbon-rich stars (C-stars) in the Small Magellanic Cloud (SMC). To achieve
this goal, we apply our recent dust growth description, coupled with a
radiative transfer code to the CSEs of C-stars evolving along the TP-AGB, for
which we compute spectra and colors. Then we compare our modeled colors in the
near- and mid-infrared (NIR and MIR) bands with the observed ones, testing
different assumptions in our dust scheme and employing several data sets of
optical constants for carbon dust available in the literature. Different
assumptions adopted in our dust scheme change the typical size of the carbon
grains produced. We constrain carbon dust properties by selecting the
combination of grain size and optical constants which best reproduces several
colors in the NIR and MIR at the same time. The different choices of optical
properties and grain size lead to differences in the NIR and MIR colors greater
than two magnitudes in some cases. We conclude that the complete set of
observed NIR and MIR colors are best reproduced by small grains, with sizes
between $\sim$0.035 and $\sim$0.12~$\mu$m, rather than by large grains between
$\sim0.2$ and $0.7$~$\mu$m. The inability of large grains to reproduce NIR and
MIR colors seems independent of the adopted optical data set. We also find a
possible trend of the grain size with mass-loss and/or carbon excess in the
CSEs of these stars.
