How can I change the size of an ODE object (a cube) according to the distance between the gripper fingers?

The currently ODE implementation doesn't let you modify the size of the object once it has been instantiated.

My suggestion would be to first create a CHAI3D cMesh for instance which you could scale as you move the gripper position of your haptic device. Once the desired size has been created, you can then instantiate the ODE object.

This being said, I would also suggest that you check the ODE documentation to see if such scaling option is possible. In which case you would simply need to scale the ODE object and the associated CHAI3D mesh.

As we are currently implementing support for the BULLET dynamics engine, we will keep your remark in mind.