Recent advances in microfabrication and nanofabrication techniques have allowed for the construction of ever-smaller sensors for the measurement of acoustic signals. The physics of sensor transduction and acoustics however place fundamental constraints on performance and limit the benefits of continual reductions in size. Key operating parameters such as sensitivity, minimum detectable signal (MDS) and resonant frequency are directly affected by device scale. Besides overall scale, relative sensor geometry (e.g. thickness ratio) can drastically alter performance. Through finite element and lumped element modeling, we study the effects of sensor geometry and scale on the sensitivity and noise performance of a piezoelectric microphone. The results indicate optimal relative geometric parameters for maximum performance and demonstrate the strong dependence on device geometry.