You can check out the LayoutChangedStarted event of the RadDocking. It will be thrown when the user starts a resize operation.
That said, if you want to limit the size of the containers in some way, I can also suggest checking out the ProportionalStackPanel attached properties.
I hope you find this helpful.
Get quickly onboarded and successful
with your Telerik and/or Kendo UI products with the Virtual Classroom free technical training, available to all active customers. Learn More.