There are a number of ways to customize the look and feel of your RadDocking control. This topic covers some of the most useful of these properties.
Set relative size of nested RadPaneGroups using the attached property ProportionalStackPanel.RelativeSize.
Use RadSplitContainer to split your panes according to your needs.
Change the orientation of your RadSplitContainer to Horizontal or Vertical using its Orientation property as shown here.
Control the initial docking positions of your panes via the InitialPosition property of the RadSplitContainer class. Choose between DockedLeft, DockedRight, DockedTop, DockedBottom, FloatingDockable and FloatingOnly. To learn more read this.