In this article, we propose a method for the optimal dimensional synthesis of planar parallel manipulators covering a specified workspace. The proposed method aims to reduce the dimensions of the links, while maximizing the dexterity index over the workspace and it improves the rate of forces transmission and the manipulator stiffness. We study the stiffness of the manipulator through the analysis of deformations based on the finite elements method. The optimization is performed using genetic algorithms, particle swarm optimization and a hybrid method based on the combination of both methods. As study cases, we apply the propose method to the dimensional synthesis of two planar parallel manipulators. Compare to the results obtained analyzing a manipulator with links length within the design space, yet not optimal, the obtained manipulators through the proposed method meet the requirements of workspace while also showing an improved performance in terms of dexterity, forces transmission and stiffness.