控件的大小绑定到上层容器的大小
<Grid Loaded="Grid_Loaded_1">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="125"/>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="50"/>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="50"/>
<RowDefinition Height="*"/>
</Grid.RowDefinitions>
<StackPanel Grid.Row="1" Grid.Column="1" Grid.ColumnSpan="2"
Orientation="Horizontal" >
<Grid Width="{Binding RelativeSource={RelativeSourceMode=FindAncestor,AncestorLevel=1,AncestorType={x:Type Grid}}, Path=Width }"
Height="248"
Background="Blue" >
</Grid>
<grid>...</grid>
</StackPanel>
<Grid>
