This paper presents a circular piezoelectric unimorph with the edge clamped for the use of ultrasound transducer engineering. The unimorph consists of one PZT layer and one elastic layer, and the diameter of the PZT layer is a crucial factor on its performance. By modeling and analyzing the flexural motions with first order lamination theory, the lateral size of the PZT layer is optimized. The size optimization is based on the maximum displacement of the central part and the effective electromechanical coupling coefficient. The numerical solutions show that vibration performance can be optimized by adjusting the size ratio of the PZT film and Al film.