Synthesis of robots may be decomposed into two processes: {\em structural synthesis} (determine the general arrangement of the mechanical structure such as the type and number of joints and the way they will be connected) and {\em dimensional synthesis} (determine the length of the links, the axis and location of the joints, the necessary maximal joint forces/torques,$\ldots$). The performances that may be obtained for a robot are drastically dependent on both synthesis. Although for serial robots general trends may be derived only from the structure a realistic comparison between two different structures may only be made after a careful dimensional synthesis and this is even more so for closed-loop robot (such as parallel robots). We will present a dimensional synthesis approach based on the design requirements that allows one to obtain almost all feasible design solutions that are guaranteed to satisfy the requirements, even taking into account manufacturing tolerances. Practical examples of 6-DOF robot design will be presented.