We developed a theoretical model for the calculation of the main characteristics of pyroelectric response of pyroactive multilayer structures, which are basic elements of modern pyroelectric detectors of radiation and other transducers.In the framework of the proposed model, the dependences of the pyroelectric response frequency spectra on the ferroelectric film thickness have been investigated. It was found that a pronounced extrinsic size effect is inherent to the pyroelectric response frequency spectra, such as the strong thickness dependence of the response maximal value and its frequency position.Thickness dependences of the frequency position of the pyroelectric response maximum and the optimal thickness of the pyroactive ferroelectric film are calculated. The frequency spectra of the pyroelectric current and voltage calculated theoretically are in quantitative agreement with experimental ones for Al/P(VDF-TrFE)/Ti/Si pyroactive structure.Developed procedure is important for improving the sensitivity of pyroelectric devices.Obtained analytical expressions allow us to optimize the performances of pyroactive elements using the existence of the size effect, which depend on pyroactive layer and substrate thicknesses, as well as on the conditions of heat transfer and exchange at corresponding boundaries of the pyroactive structure.