Traditional brightness-based spatial target size inversion methods require the combination of detector response, and the response error can affect the accuracy of the inversion. This paper proposes a method that introduces cooperative target brightness information to assist in size inversion. The brightness information of cooperative targets is compared with spatial targets to eliminate the detector response with larger errors and then is transformed into more accurate position information of cooperative targets, thereby improving the accuracy of size inversion. This paper first establishes a spatial target observation model based on the brightness and position information of cooperative targets. Then based on the radiometry, the analytical formula for spatial target size inversion is derived, and the calculation error is given. Finally, the model is validated with an example. The result shows that the size inversion accuracy based on cooperative target brightness information is improved by about 46.9% compared with traditional methods.