  We consider constraints on the planetesimal population residing in the disks
of AU Microscopii, Beta Pictoris and Fomalhaut taking into account their
observed thicknesses and normal disk opacities. We estimate that bodies of
radius 5, 180 and 70 km are responsible for initiating the collisional cascade
accounting for the dust production for AU-Mic, Beta-Pic and Fomalhaut's disks,
respectively, at break radii from the star where their surface brightness
profiles change slope. Larger bodies, of radius 1000km and with surface density
of order 0.01 g/cm^2, are required to explain the thickness of these disks
assuming that they are heated by gravitational stirring. A comparison between
the densities of the two sizes suggests the size distribution in the largest
bodies is flatter than that observed in the Kuiper belt. AU Mic's disk requires
the shallowest size distribution for bodies with radius greater than 10km
suggesting that the disk contains planetary embryos experiencing a stage of
runaway growth.
