  Context: The size of the constituent particles (monomers) of dust aggregates
is one of the most uncertain parameters directly affecting collisional growth
of aggregates in planet-forming disks. Despite its importance, the monomer size
has not yet been meaningfully constrained by disk observations. Aims: We
attempt to derive the monomer size from optical and near-infrared (IR)
polarimetric observations of planet-forming disks. Methods: We perform a
comprehensive parameter survey on the degree of linear polarization of light
scattered by dust aggregates, using an exact numerical method called the
$T$-matrix method. We investigate the effect of the monomer size, aggregate
size, porosity, and composition on the degree of polarization. The obtained
results are then compared with observed polarization fractions of several
planet-forming disks at optical and near-IR wavelengths. Results: It is shown
that the degree of polarization of aggregates depends sensitively on the
monomer size unless the monomer size parameter is smaller than one or two.
Comparing the simulation results with the disk observations, we find that the
monomer radius is no greater than $0.4~\mu$m. The inferred monomer size is
therefore similar to subunit sizes of the solar system dust aggregates and the
maximum size of interstellar grains. Conclusions: Optical and near-IR
quantitative polarimetry will provide observational grounds on the initial
conditions for dust coagulation and thereby planetesimal formation in
planet-forming disks.
