  We report a Monte Carlo simulation study of the properties of highly
asymmetric binary hard sphere mixtures. This system is treated within an
effective fluid approximation in which the large particles interact through a
depletion potential (R. Roth {\em et al}, Phys. Rev. E{\bf 62} 5360 (2000))
designed to capture the effects of a virtual sea of small particles. We
generalize this depletion potential to include the effects of explicit size
dispersity in the large particles and consider the case in which the particle
diameters are distributed according to a Schulz form having degree of
polydispersity 14%. The resulting alteration (with respect to the monodisperse
limit) of the metastable fluid-fluid critical point parameters is determined
for two values of the ratio of the diameters of the small and large particles:
$q\equiv\sigma_s/\bar\sigma_b=0.1$ and $q=0.05$. We find that inclusion of
polydispersity moves the critical point to lower reservoir volume fractions of
the small particles and high volume fractions of the large ones. The estimated
critical point parameters are found to be in good agreement with those
predicted by a generalized corresponding states argument which provides a link
to the known critical adhesion parameter of the adhesive hard sphere model.
Finite-size scaling estimates of the cluster percolation line in the one phase
fluid region indicate that inclusion of polydispersity moves the critical point
deeper into the percolating regime. This suggests that phase separation is more
likely to be preempted by dynamical arrest in polydisperse systems.
