In shallow water environments, target scattered echoes are affected by multipath interference and boundary reflections, which can easily cause the target scattering features to be masked by interface reverberation and propagation effects, making them difficult to identify. This study focuses on the stable resonance features formed by the coherent superposition of the specular reflection echo and various circumferential waves. A scattered acoustic field model based on normal modes is established for shallow water, and the range–frequency spectra of a spherical target at different depths are calculated. The formation mechanism of transverse resonance stripes is analyzed, and the quantitative relationship between the frequency spacing of the resonance spectral valleys and the target radius is derived, yielding a size estimation formula. A scaled water tank experiment is designed in which the echoes of a solid elastic sphere at multiple depths are collected to obtain the range–frequency spectra, from which the resonance spectral valley frequencies and their spacing are extracted. The results show that the frequency positions of the resonance spectral valleys exhibit good stability against target depth variations. The proposed formula achieves accurate estimation of the target's geometric scale, demonstrating that resonance frequencies have the potential for target detection and identification in complex shallow water environments.