  We investigate the distribution of instanton sizes in the framework of a
simplified model for ensembles of instantons. This model takes into account the
non-diluteness of instantons. The infrared problem for the integration over
instanton sizes is dealt with in a self-consistent manner by approximating
instanton interactions by a repulsive hard core potential. This leads to a
dynamical suppression of large instantons. The characteristic features of the
instanton size distribution are studied by means of analytic and Monte Carlo
methods. We find a power law behaviour for small sizes, consistent with the
semi-classical results. At large instanton sizes the distribution decays
exponentially. The results are compared with those from lattice simulations.
