Thank you for your interest in the new RadDock.
The layout system was completely revamped and entirely new semantic was introduced. Each SplitPanel has a member SizeInfo, which controls its size and position on the RadSplitContainer. When you have document tabstrips visible, they have SplitPanelSizeMode.Fill, which sets the underlying layout engine in a special mode, somewhat similar to the standard Windows Forms Dock layout but more extended to support nested containers and proportional shrinking whenever there is less available size than the required. So, in your case you may do the following:
|RadDock dock = new RadDock();
|ToolWindow window = new ToolWindow("My Tool Window");
|DockTabStrip strip = (DockTabStrip)window.TabStrip;
|strip.SizeInfo.AbsoluteSize = new System.Drawing.Size(100, 100);
The above code will make the toolwindow hosting tabstrip have a width of 100 pixels. To achieve different layout scenarios, you may use this approach to modify the SizeMode and set different size settings.
Please, do not hesitate to contact us if you need further assistance.
the Telerik team