A kinematic optimization method of parallel manipulators is presented in this paper. A desired workspace for a parallel manipulator is usually an essential requirement in a practical application. Additionally, a good kinematic performance and/or a relatively small physical size would be of great significance. A dexterity index is utilized to measure the kinematic performance of parallel manipulators. A method to define the physical size of a parallel manipulator is introduced. The cost function is then formulated as a linear combination of a dexterity index and a physical size measurement. The Optimization Tool Box of MATLAB is applied to solve the optimization problem. Finally, a general Stewart-Gough platform is taken as an example to specify the design methodology.