The cold main classical Kuiper Belt consists of those small solar system bodies with low orbital inclinations and orbital semi-major axes between 42.4 and 47.7~au. Various arguments suggest that these objects formed textit{in situ} and the original population has experienced minimal collisional modification since their formation. Using the Outer Solar System Origins Survey (OSSOS) ensemble sample and characterization, combined with constraints on the number of small cold classical objects from deeper surveys and supported by evidence from the Minor Planet Center catalog, we determine the absolute magnitude $H_r$ distribution of the cold classical belt from $H_rsimeq5$ to 12 (roughly diameters of 400 km to 20 km). We conclude that the cold populations size distribution exhibits an exponential cutoff at large sizes. Exponential cutoffs at large sizes are not a natural outcome of pair-wise particle accretion but exponentially tapered power-law size distributions are a feature of numerical simulations of planetesimal formation via a streaming instability. Our observation of an exponential cutoff agrees with previous observational inferences that no large objects ($D gtrsim 400$~km) exist in the cold population. Studies of the transneptunian region are providing the parameters that will enable future streaming-instability studies to determine the initial conditions of planetesimal formation in the $approx 45$~au region of the Suns protoplanetary disk.
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