The MOBOT* project aims to develop assistive devices that support elderly patients with mobility disabilities during gait and sit-to-stand (STS) transfer. The device is supposed to help such patients to maintain an independent life through mobility as well as to regain basic life skills since self-effort is encouraged. Situation-adapted optimal assistance will be provided by the device both proactively and adaptively through behavior-based and context-aware robot control based on multimodal action recognition. The design of such a device, including the choice of actuators, which can provide the desired level of support without losing balance, is a challenging task. For these design choices, in particular STS motions have to be considered since they are more demanding for both the patient and the device. In this paper, we present a model-based optimization approach to this design task, using direct multiple shooting optimal control methods. They are applied to simultaneously determine optimal states and controls as well as the optimal mechanical design parameters of the assistive device that provide optimal STS assistance to subjects from a range of body heights and weights that represents the MOBOT target group.