Just add a ComponentListener to the Component that represents the Dockable. Like this:
CDockable dockable = ...
Dockable intern = dockable.intern()
intern.getComponent().addComponentListener( new MyComponentListener( ... ) )
DockingFrames does not have a special method for tracking the size of a Dockable.
You might also be able to get some results by adding a "CDockableLocationListener" to the "CDockable". But that listener is intended to be notified if the position (e.g. the parent) of a Dockable changes - notifications of changed size is a side effect at best, and not guaranteed.