size load motors for torque loop tuning

[GmEE]

Is there any general rule of thumb in torque loop tuning on how to size the load/dyno motor to the tested motor? What if the size of the tested motors vary by orders of magnitude, i.e. 10 to 1000 Nm peak torque? Also, can anyone recommend a good handbook for turque loop tuning? In case its not obvious, I'm just starting out.