To improve the mechanical structure of cable-driven parallel manipulators (CDPMs), the size of the moving platform is optimized based on the stiffness characteristics. First, the relationship between the stability and the static stiffness of CDPMs is analyzed, and a way of judging whether CDPMs are stable at a given pose is proposed. Therefore, the space formed by all stable poses is defined as the stable workspace. Second, the effects of two different shapes of the moving platform in different sizes on the stable workspace are investigated numerically. Furthermore, by maximizing the stable workspace, the size of the moving platform is optimized using the grouped coordinate descent method. Finally, the simulation experiments are implemented on a six-DOF spatial CDPM with eight cables. The simulation results indicate that the shape and size of the moving platform both have effects on the stable workspace, and the volume of the stable workspace can be enlarged by optimizing the size of the moving platform in the two different shapes.