  The gravitational instability of a dust layer is one of the scenarios for
planetesimal formation. If the density of a dust layer becomes sufficiently
high as a result of the sedimentation of dust grains toward the midplane of a
protoplanetary disk, the layer becomes gravitationally unstable and
spontaneously fragments into planetesimals. Using a shearing box method, we
performed local $N$-body simulations of gravitational instability of a dust
layer and subsequent coagulation without gas and investigated the basic
formation process of planetesimals. In this paper, we adopted the accretion
model as a collision model. A gravitationally bound pair of particles is
replaced by a single particle with the total mass of the pair. This accretion
model enables us to perform long-term and large-scale calculations. We
confirmed that the formation process of planetesimals is the same as that in
the previous paper with the rubble pile models. The formation process is
divided into three stages: the formation of non-axisymmetric structures, the
creation of planetesimal seeds, and their collisional growth. We investigated
the dependence of the planetesimal mass on the simulation domain size. We found
that the mean mass of planetesimals formed in simulations is proportional to
$L_y^{3/2}$, where $L_y$ is the size of the computational domain in the
direction of rotation. However, the mean mass of planetesimals is independent
of $L_x$, where $L_x$ is the size of the computational domain in the radial
direction if $L_x$ is sufficiently large. We presented the estimation formula
of the planetesimal mass taking into account the simulation domain size.
