The aim of this study is to compare the performance of Optimized Link State Routing (OLSR), Ad hoc On-demand Distance Vector (AODV) and Greedy Perimeter Stateless Routing (GPSR) routing protocols with respect to network size and density. Each of these protocols represents the three categories of MANET routing protocols which are proactive, reactive and geographical routing protocol, respectively. The evaluation was done through simulation and the performance was measured in terms of throughput, average End-to-End (E2E) delay, Packet Delivery Fraction (PDF) and Normalized Routing Load (NRL). The results of the simulations show that the GPSR protocol is superior to OLSR and AODV in most cases. The results also show that throughput, end-to-end delay and packet delivery fraction are largely affected by the network size, while normalized routing load is largely affected by the number of nodes in the network.