Optimal dimensioning is a fundamental problem in the design of Parallel Manipulators (PMs). However, this problem is revealed to be very hard to solve because many PM performance criteria are antagonistic. In addition, the required technical specifications can be difficult to obtain because they are usually dependent on the end-effector posture and the task to be performed. In this paper, we present an overview of the different approaches used in the optimal design of PMs, as well as the main difficulties encountered. Technical solutions are proposed to solve the problem of optimal dimensional design of PMs. In addition, a seven-stage optimal design methodology for PMs is proposed. Finally, we present an illustrative application of the methodology developed for a 5R parallel manipulator with two degrees of freedom.