Photoacoustic tomography (PAT) is an emerging biomedical imaging technology. The conventional PAT system setup, the ring-distribution ultrasonic transducer (UT) array only receives PA signals from fixed region of interest (ROI) that limit the flexibility for imaging objects with different sizes. In this paper, a size-adjustable PAT system with sectorial ultrasonic transducer array (SUTA) is proposed. The four SUTAs can adjust the imaging ROI’s radius and improves the size-adjustable capability of PAT system. The feasibility of the size-adjustable PAT imaging is firstly verified by the simulation results. Moreover, a 3-D printed vascular phantom is imaged by the proposed PAT system validating its feasibility very well.