In order to optimize their maintenance costs, airlines are very interested in the condition based maintenance approach. For this purpose, structural health data coming from the monitoring of structural subcomponents and in particular their residual useful life (RUL) are used. If a damage is detected in the part, the evolution of its size will strongly influence the RUL. It is therefore essential to have a tool to monitor the size of defects in structures in a reliable manner. In this article, we propose an analytical scattering model that deals with piezoelectric transducers in acting and sensing mode. The actuator generates Lamb waves which are reflected by an inhomogeneity in the material and captured by the PZT in sensor mode. By specifying the material parameters and the input signal, the model predicts the output signal. The theoretical model is successfully validated on simulation data. Finally, the model is used to perform damage size quantification.