Eddy current testing (ECT) has three main tasks: detection, location, and characterization of defects. The characterization task, which means the ability to find the geometrical characteristics of the defect, is still in the research domain, although in many industrial applications this task has to be carried out with good accuracy to allow reliable acceptance or rejection decision indispensable to save costs and even human lives. This paper proposes an ECT measurement method that allows the reliable estimation of the geometrical characteristics of thin defects (i.e., length, height, and depth) by using a combination of a multifrequency excitation and an optimized support vector machine for regression. The proposed solution is tested on real specimens with known cracks by using a suitable measurement setup comprising a giant magnetoresistance-based biaxial ECT probe.