Evidences show that besides their incredible performance, private BitTorrent communities which employ credit-based contribution incentives can also confront with system seize-up. Although the sustainability of these communities can be established in a wide range of system parameters, it is still not fully understood under what initial conditions can this be assured. In this paper, with the usage of extensive simulation results, we investigate the effects of the file size and the impact of initial credit amount distribution in order to deduce further conditions of sustainability.