A Mobile Ad hoc NETwork (MANET) consists of mobile nodes which are move arbitrarily within the specified range. MANETs can be easily organized without the aid of any infrastructure. Due to the nature of shared resources performance of routing protocols and congestion control is a critical issue in mobile ad hoc networks. In this paper, the authors formulate different simulations using NS2 for ad hoc networks of different sizes. This paper analyzed impact of simulation area on the performance of some commonly used routing protocols under the radio propagation models two-ray and shadowing.