This paper describes an optimization method of the top electrode size for a thin film matrix ultrasonic transducer (M-UT) in the frequency range above 200 MHz. The goal of this work is to design an optimal top electrode size for an M-UT providing the maximal output peak–peak voltage (VPP) and the maximal signal-to-noise ratio (SNR) without additional electrical impedance matching. In order to reduce the complexity of the M-UT with more than 1000 elements, an intrinsic matching by electrode size variation is necessary. However, the size of a single element top electrode for an M-UT is related to the number of elements within a targeted sensor area, VPP and SNR of the transducer. In this paper, varying the active area of the top electrode from 0.09 to 25 mm2 shows that for an Al–AlN–Al on silicon wafer configuration connected with a JSR Ultrasonics pulser/receiver (50 ohms), the optimal electrode size is 1 mm2. With the optimal size electrode, the maximum output VPP of 0.08 V and the SNR of 42.93 dB are achieved at the resonance frequency of 225 MHz, and the bandwidth is16.21 MHz.