Initial observational data from the impact of fragments of Comet Shoemaker-Levy 9 (SL9) are compared with smoothed particle hydrodynamic (SPH) calculations to determine their pre-impact diameters and the equivalent diameter of the SL9 progenitor. Diameters (solid ice) of 2.0¿0.1, 2.0¿0.05, 2.1¿0.04 and 1.9¿0.05 km for fragments A, E, G1, and W are obtained from impact-induced plume heights from the Hubble Space Telescope (HST) data. Applying these values to scale apparent diameters for the balance of 18 SL fragments in Weaver et al.'s [1995> catalog of 22 objects yields a SL9 progenitor diameter of 5.0¿1.8 km. This corresponds to total impact energy of 1.2 (+1.8-0.8)¿1030 erg. Such an energetic event occurs on Jupiter and Earth at least every 4,900+4,700, -2,700, and ~0.5¿108 years, respectively. ¿ American Geophysical Union 1995