  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 $N = 2500\pm500$ 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.
