Hi Sopheap Ly,
Currently, the functionality you're looking for is not supported for the Fill mode of docking window. A Dock window can be set to fixed width.
In order to do that, set Size property to desired width inside the handler of its Resize event:
|void dockPanel2_Resize(object sender, EventArgs e)
| dockPanel2.Size = new Size(200,0);
The new API for fixed docking window will be added to our future releases.
If you have any questions, please contact me.
the Telerik team