We compare the observed size distribution of circum stellar disks in the Orion Trapezium cluster with the results of $N$-body simulations in which we incorporated an heuristic prescription for the evolution of these disks. In our simulations, the sizes of stellar disks are affected by close encounters with other stars (with disks). We find that the observed distribution of disk sizes in the Orion Trapezium cluster is excellently reproduced by truncation due to dynamical encounters alone. The observed distribution appears to be a sensitive measure of the past dynamical history of the cluster, and therewith on the conditions of the cluster at birth. The best comparison between the observed disk size distribution and the simulated distribution is realized with a cluster of $N = 2500pm500$ stars with a half-mass radius of about 0.5,pc in virial equilibrium (with a virial ratio of $Q = 0.5$, or somewhat colder $Q simeq 0.3$), and with a density structure according to a fractal dimension of $F simeq 1.6$. Simulations with these parameters reproduce the observed distribution of circum stellar disks in about 0.2--0.5,Myr.
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