To dock the IDockable window like DockPanel you can use SetDock method of DockingManager. After you dock the panel you can resize it with splitter or use Size property of DockPanel object.
| DockPanel dockPanel = new DockPanel();
| dockPanel.Text = "My Dock Panel";
| dockingManager1.SetDock(dockPanel, DockPosition.Left);
| dockPanel.Size = new Size(200, 0);
I hope this helps with your question. Should you have further questions, don't hesitate to ask us.
the Telerik team