Lamb waves have potential applications in structural health monitoring; however, most studies on delamination damage are limited to delamination localization and accuracy improvement, and few studies on delamination size measurement exist. This study investigates the effects of the delamination defect size on Lamb waves of different wavelengths in carbon fiber composite panels. The interaction between the A0 mode of linear FM Lamb waves and different delamination sizes is investigated via finite element simulation, and the degree of perturbation is found to increase monotonically with the delamination size when the wavelength is larger than the delamination size and that the degree of perturbation is in an oscillatory state when the wavelength is smaller than the delamination size. Facilitated by this phenomenon, a method is proposed to detect delamination defects in carbon fiber boards. A signal with a larger wavelength is selected as the excitation, and the damage index is used to portray the degree of disturbance. Moreover, the damage index is integrated into the imaging algorithm so that the imaging results can reflect the delamination size information.