In this paper a new method for determination and optimization of the workspace of parallel manipulators is presented. The proposed method is based on a geometrical approach, and offers the possibility to generate automatically the workspace in a CAD environment. Thus, the relationship between the geometrical design parameters of the parallel manipulator and its workspace can be analyzed without difficulty. Optimization problem considered in this paper consists in determining the dimensions of a parallel manipulator having the closest workspace to a prescribed task region. Finally, numerical applications of two types of planar parallel manipulators are presented in order to illustrate the proposed approach.