C# WPF Auto-Resize when Maximized button is pressed.

Question

attached is my code... I have this... .but still does not work, why?

        <Grid.RowDefinitions>
            <RowDefinition Height="10*"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="6000"></ColumnDefinition>
            <ColumnDefinition Width="Auto"/>
            <ColumnDefinition Width="*"></ColumnDefinition>
        </Grid.ColumnDefinitions>

  <GridSplitter Grid.Row="1" Grid.Column="1" Width="5" HorizontalAlignment="Center" VerticalAlignment="Stretch"/>