  The trans-Neptunian object 2014 MU69, named Arrokoth, is the most recent
evidence that planetesimals did not form by successive collisions of smaller
objects, but by the direct gravitational collapse of a pebble cloud. But what
process sets the physical scales on which this collapse may occur? Star
formation has the Jeans mass, that is when gravity is stronger than thermal
pressure, helping us to understand the mass of our sun. But what controls mass
and size in the case of planetesimal formation? Both asteroids and Kuiper belt
objects show a kink in their size distribution at 100 km. Here we derive a
gravitational collapse criterion for a pebble cloud to fragment to
planetesimals, showing that a critical mass is needed for the clump to overcome
turbulent diffusion. We successfully tested the validity of this criterion in
direct numerical simulations of planetesimal formation triggered by the
streaming instability. Our result can, therefore, explain the sizes for
planetesimals found forming in streaming instability simulations in the
literature, while not addressing the detailed size distribution. We find that
the observed characteristic diameters of $\sim$ 100 km correspond to the
critical mass of a pebble cloud set by the strength of turbulent diffusion
stemming from streaming instability for a wide region of a solar nebula model
from 2 - 60 au, with a tendency to allow for smaller objects at distances
beyond and at late times, when the nebula gas gets depleted.
