The initial size distribution of cometesimals (blue) resulting from ~3 Myr of hierarchical agglomeration in the solar nebula combined with selective accretion by TNOs. It is a q = 3.47 power law at D ≥ 1 km but flattens at 0.1 ≤ D ≤ 1 km. The size distribution evolves through low-velocity (u = 40 m s^-1) mergers during the first 25 Myr of the gas–free primordial disk (red) according to Eq. (9).