This sample bind a textblock width to it's parent's actual width.
<Grid>
<ListView>
<ListView.ItemTemplate>
<DataTemplate>
<TextBlock Text="{Binding Mode=OneWay}" Background="LightGreen" Foreground="Red"
Width="{Binding ActualWidth,
RelativeSource={RelativeSource AncestorType={x:Type ListView}}}" />
</DataTemplate>
</ListView.ItemTemplate>
<sys:String>Merry</sys:String>
<sys:String>Christmas</sys:String>
</ListView>
</Grid>