Clamp-on flow meters need a-priori knowledge of pipe geometry and material properties for their operation. Estimation of these properties may limit the accuracy of flow measurements. We are currently investigating the use of clamp-on flow meters based on matrix transducer arrays. Besides the benefits for flow measurement, these can be used for a-priori measurement of the required pipe properties. In the axial direction, average longitudinal wave speed and thickness of the pipe can be obtained from the dispersion curves of the Lamb waves in the pipe wall. In the circumferential direction, the arrival time of the fastest Lamb wave mode can be used to measure the pipe diameter. In our presentation, this method is discussed and proof of principle is provided through FEM simulations and measurements. The setup consisted of two pipes (outer diameters 64.5 mm and 93 mm), with two transducers (1 MHz) on top. The arrival time of the fastest Lamb wave mode was measured. For the outer diameter, the error between simulation and measurement was below 1% (small diameter) and below 3% (large diameter). The results suggest that further exploration of the unique possibilities of matrix transducers to accurately measure pipe parameters is opportune.