The use of Focal Plane Arrays (FPA) as feeds for reflector antennas constitutes a new-generation solution for future radio telescope systems. In this paper, we investigate one of two main factors affecting the overall performance of such FPA-fed reflector systems, which is characterized by the total aperture efficiency and the bandwidth over the Field-of-View. These are strongly influenced by the properties of both the reflector geometry and the FPA feed. The parameters that define the latter are the number of elements comprising the array, and the size of each element. The goal here is to provide insights into how these array parameters affect the total aperture efficiency and bandwidth. The second factor, namely, system noise temperature is not studied in this paper and will be addressed in a different publication. In the process of our study, an optimized FPA geometry has been determined for each of several considered reflector cases. The array element that is used to perform this study is the open-ended aperture of a hard rectangular waveguide (HRW). The computed results are obtained from a computer code based on a rigorous full-wave analysis that takes the important mutual coupling within the array into account.