This paper presents a method for rapid quantification of long artificial cracks using a GMR-based eddy current sensor. The quantification is achieved simultaneously through a proposed phenomenological relationship between the crack's dimension and two parameters extracted from the GMR output signal. The results show that the proposed method is valid and advantageous when a quick and reliable estimation of the crack's dimension is necessary. A criterion taking into consideration the filling factor of the excitation coil is proposed in order to improve the system's sensitivity.