A parametric 2D Finite-Element scheme is developed to locate and size strip-like defects in elastic or viscoelastic, isotropic or anisotropic material plates. The reflection and transmission coefficients produced by mode conversion phenomenon when a pure incident Lamb wave mode is sent towards the defect, are used as input data for the inversion process. The inversion process consists in quantifying one or two unknown parameters representing the geometry of the defect. A finite Element based model is used to simulate the Lamb wave scattering for various values of the aimed parameters, and specific post-processing based on the Shkerdin's orthogonality relation is applied to predict the needed reflection and transmission coefficients. The target is known reflection and transmission coefficients initially obtained from an experiment, which can be either a numerical experiment for validation purposes, or proper measurements on a sample for NDE application purposes. The inversion process is stopped when the predicted reflection and transmission coefficients fit at best those obtained from the initial experiment. In this preliminary study, different geometrical parameters have successfully been quantified for several types of defects in various waveguide : (1) the depth and width of a notch at the surface of an Aluminium or Perspex plate, (2) the through-thickness position and radius of a circular hole inside an Aluminium plate, (3) the through-thickness position and width of a delamination in a composite plate.