Contention-based geographic relay selection for cooperative networks has been proposed as a state-free distributed scheme to efficiently select the best relay within a geographic area. The a priori choices for the relaying area cause different effects on the overall network performance. In this paper, we focus on the fundamental problem of predefining the relaying area and investigate its impact on the network performance. The previous work named CoopGeo, which is a cross-layer designed protocol including the relay and next hop selections, suggested the Reuleaux triangle as the relaying area, so as to avoid the hidden node problem in relay selection. However, it also leads to a certain loss of potential relays beneficial to the cooperation. Following the framework of CoopGeo, we develop a series of mathematical expressions for the relaying area, based on the average symbol error rate (SER) performance requirement. Further, the relaying area is derived and modeled as an ellipse-shaped region. Simulation results show that the CoopGeo using the proposed relaying area improves the network performance in terms of the delivery failure rate. The relaying area formulation and derivations provide a guideline regarding the design of the relaying area.