Also make sure the STL uses mm as coordinate system units (or scale it using a transforms module).
Note that you can also animate the robot (simulate rotation around each joint) if you load each section of the robot as a separate model, with the rotation joint in the origin. You just need to create a hierarchy of transforms in Transforms module and add each model at the appropriate level in the transform tree.
I’ve uploaded a short video of ABB IRB 120 robot visualization in Slicer:
You can download the Slicer scene from here.