In order to measure lightning flashes, different measuring systems have been developed and validated in the last 50 years. Between them, indirect measuring systems, based on the use of parallel-plate antennas to measure the electric field radiated by the electrical discharges, are one of the most common methods due to their simplicity and their low cost. This paper analyzes the influence of the shape and the physical dimensions of the parallel-plate antenna used for the indirect measurement of cloud-to-cloud and cloud-to-ground lightning flashes. The tests were carried out using simulations where the geometric shape of the antenna, the characteristics of the neighboring structures (including the electric permittivity and the electric conductivity) and the behavior of different materials (present in the electromagnetic environment) were examined. In addition, the correction factors that must be included for a complete characterization of the electric field measuring system were obtained.