You can set width globally for all LayoutDocument
in DockingManager
resources:
<avalonDock:DockingManager x:Name="DockManager" AllowMixedOrientation="True">
<avalonDock:DockingManager.Resources>
<Style TargetType="{x:Type avalonDock:LayoutDocumentTabItem}">
<Setter Property="Width" Value="120" />
</Style>
</avalonDock:DockingManager.Resources>
<avalonDock:DockingManager.Theme>
<avalonDock:AeroTheme/>
</avalonDock:DockingManager.Theme>
...