No matter how far I zoom in or out, those handlers should always remain the same size.
Regarding this, you could add them as a scalable node and calculate their scale based on your camera zoom so that they remain the same.
Regarding the handlers themselves, you'd have to program a bit of logic to do the mapping of handlers' position to the resizeable node's size. You could export some bools to define which size axis that particular handle modifies, and add the logic in the handler's class itself to do the modifications to its parent.
Also, you'd need a bit of logic to move the connected nodes if you need to. For example, if I move the top left handle upwards, the top right handle must move upwards also.
I can't think of any more complexity than what I mentioned, although I never tried this before so I don't know if the effect would be the one desired. Let me know if I can be of more help, maybe go a bit more in depth.
Hope it helps.
If you need further explanation, I can try to go a bit more in-depth.