The design of compact and inexpensive mechanical drives and gearboxes can take advantage from the use of custom rolling bearings in which the rolling elements are directly in contact with the shaft and the housing containing them. The custom construction is particularly convenient for the case of roller bearings, either cylindrical or tapered, because these elements are easily manufactured and provide a remarkable loading capacity. Starting from the standard load rating equations available in the literature (ISO standards), this paper develops a step-by-step design procedure for the geometric optimization of radial bearings with cylindrical rollers. The procedure includes constraints on the geometry and leads to the optimal bearing with the maximum static and dynamic load ratings compatible with the space available.