Off-axis three-mirror (TMA) systems are based on coaxial three mirror systems, which can avoid the obstructions through field off-axis and aperture off-axis. Traditional methods obtain coaxial system parameters such as structure intervals by calculating obscuration ratios and amplifications three mirror, which fails to limit the physical size of system directly. Based on traditional method, a new method is proposed to calculate coaxial configuration parameters by offering the interval of three mirror and distances between three mirrors and image plane. Configuration parameters are initialized by Matlab and further optimized via Zemax. According to the existence of an intermediate image or not, Cook TMA and Wetherell TMA are designed with effective focal length of 1500 mm and entrance pupil of 250 mm. Experimental results demonstrate that the modulation transform functions at the spatial frequency of 50 lp/mm are above 0.6, and the root-mean-square radii of diffusion plaques are below 5 μm . The proposed method can determine system size and the image quality meets the requirements.