A conventional axion search experiment utilizes microwave resonant cavities, where axions are converted into photons under a strong magnetic field. Optimized cavity dimension is essential to enhance signal power from the axion-to-photon coupling, to broaden the frequency range, to minimize mode crossings, etc. An extensive study has been performed to optimize the dimension of a cavity and frequency tuning system using the COMSOL multiphysics simulation software. We introduce a figure of merit for this purpose, and present the results from the simulation study.