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We show, by comparing observations with theoretical models, that the observedKuiper Belt size distribution is well matched by coagulation models, whichstart from an initial planetesimal population with radii of about 1km, andsubsequent collisional evolution. We find that the observed size distributionfor R > 30km has not been modified by collisional evolution over the age of thesolar system, and that the size distribution below R ~ 30km has been modifiedby collisions and that its slope is well matched by collisional evolutionmodels that use published strength laws. We investigate in detail the resultingsize distribution of bodies ranging from 0.01km to 30km and find that its slopechanges several times as a function of radius before approaching the expectedvalue for an equilibrium collisional cascade of material strength dominatedbodies for R < 0.1km. Compared to a single power law size distribution thatwould span the whole range from 0.01km to 30km, we find a strong deficit ofbodies around R ~10km and a strong excess of bodies around 2km. This deficitand excess are caused by an excess mass in small planetesimals in the km sizerange that was left over from the runaway growth phase and that leaves asignature in the size distribution that is not erased after 4.5 Gyrs ofcollisional evolution. Observations of the small KBO size distribution cantherefore test if large KBOs grew as a result of runaway growth and constrainthe initial planetesimal sizes. We find that results from recent KBOoccultation surveys and the observed KBO size distribution can be best matchedby an initial planetesimal population that contained about equal mass perlogarithmic mass bin in bodies ranging from 0.4km to 4km in radius. Inaddition, we find that we cannot match the observed KBO size distribution ifmost of the planetesimal mass was contained in bodies that were 10km in radiusor larger.

Initial Planetesimal Sizes and the Size Distribution of Small Kuiper Belt Objects