I'm not a mechanical engineer, but even a novice can see you need the following before you can start guessing:
1. Size of 50kg object (is it a ball, is it a 3 meter long bar?)
2. Length of arm/segments, including end effector required
3. Approximate range of movement for each DOF
4. Speed required from start point to end point(s)
5. Size/ratio of harmonic drives to be used (determines max torque/speed per joint)
6. Electrical power available to the system (volts/amps available)
In the absence of the above you can't really figure it out. Might as well just find a commercial robot arm with a 50kg payload and copy the thing.