In this paper we discuss cosmological N-Body/SPH simulations on parallel computing systems with distributed memory using GADGET-2. GADGET-2 (GAlaxies with Dark matter and Gas intEracT) is a novel cosmological simulation code, written in C++ and publicly available, developed by Volker Springel at the Max-Planck-Institute for Astrophysics in Munchen, Germany as an improved version of GADGET. It is a massively parallel code that uses an explicit communication model implemented with the standardized MPI communication interface. Our contribution consists of a scalability and performance analysis, done on different types of simulations ranging from thousands to hundreds of millions of particles. The size of the simulation is limited by the amount of available physical memory (RAM) when provided with sufficient computing time. Based on our study, we propose an inferior and superior limit to astrophysical simulations than can be run on a specific parallel computing system while maintaining a specific efficiency. We also outline a trend stating the computational resources that are necessary depending on the size of the simulation.