  The streaming instability is a mechanism to concentrate solid particles into
overdense filaments that undergo gravitational collapse and form planetesimals.
However, it remains unclear how the initial mass function of these
planetesimals depends on the box dimensions of numerical simulations. To
resolve this, we perform simulations of planetesimal formation with the largest
box dimensions to date, allowing planetesimals to form simultaneously in
multiple filaments that can only emerge within such large simulation boxes. In
our simulations, planetesimals with sizes between 80 km and several hundred
kilometers form. We find that a power law with a rather shallow exponential
cutoff at the high-mass end represents the cumulative birth mass function
better than an integrated power law. The steepness of the exponential cutoff is
largely independent of box dimensions and resolution, while the exponent of the
power law is not constrained at the resolutions we employ. Moreover, we find
that the characteristic mass scale of the exponential cutoff correlates with
the mass budget in each filament. Together with previous studies of
high-resolution simulations with small box domains, our results therefore imply
that the cumulative birth mass function of planetesimals is consistent with an
exponentially tapered power law with a power-law exponent of approximately -1.6
and a steepness of the exponential cutoff in the range of 0.3-0.4.
