Photoacoustic tomography (PAT) is an emerging biomedical imaging technology. Many kinds of PAT imaging modalities are applied in various biomedical applications. However, for the conventional PAT system setup, the ringdistribution 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. Four 70.8-degree 32- elements SUTAs with different center frequency are placed around the target to receive 128 channels' PA signals. The four SUTAs can adjust the imaging ROI's radius ranging from 50 mm to 90 mm, which greatly improves the size-adjustable capability of PAT system. The four SUTAs received signals are used to recover the original PA signals based on physical model accordingly. To prove the concept, the feasibility of the size- adjustable PAT imaging is firstly verified by the simulation results using MATLAB k-wave toolbox. Moreover, a 3-D printed vascular phantom is imaged by the proposed PAT system validating its feasibility very well.