  Planetary cores are thought to form in proto-planetary disks via the growth
of dusty solid material. However, it is unclear how early this process begins.
We study the physical structure and grain growth in the edge-on disk that
surrounds the ~1 Myr old low-mass (~0.55 Msun) protostar embedded in the Bok
Globule CB26 to examine how much grain growth has already occurred in the
protostellar phase. We combine the SED between 0.9 ${\mu}$m and 6.4 cm with
high angular resolution continuum maps at 1.3, 2.9, and 8.1 mm, and use the
radiative transfer code RADMC-3D to conduct a detailed modelling of the dust
emission from the disk and envelope of CB 26. We infer inner and outer disk
radii of around 16 au and 172$\pm$22 au, respectively. The total gas mass in
the disk is ~0.076 Msun, which amounts to ~14% of the mass of the central star.
The inner disk contains a compact free-free emission region, which could be
related to either a jet or a photoevaporation region. The thermal dust emission
from the outer disk is optically thin at mm wavelengths, while the emission
from the inner disk midplane is moderately optically thick. Our best-fit
radiative transfer models indicate that the dust grains in the disk have
already grown to pebbles with diameters of the order of 10 cm in size. Residual
8.1 mm emission suggests the presence of even larger particles in the inner
disk. For the optically thin mm dust emission from the outer disk, we derive a
mean opacity slope of 0.6$\pm$0.4, which is consistent with the presence of
large dust grains. The presence of cm-sized bodies in the CB 26 disk indicates
that solids grow rapidly already during the first million years in a
protostellar disk. It is thus possible that Class II disks are already seeded
with large particles and may contain even planetesimals.
