<Window Height="1140" Width="1450">
    <Grid Margin="0" Background="White">
        <Grid>
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="250" />
                <ColumnDefinition Width="*" />
            </Grid.ColumnDefinitions>
            <Grid.RowDefinitions>
                <RowDefinition Height="*" />
            </Grid.RowDefinitions>
            <Grid Background="#FFEEEEEE" Grid.Column="0" Grid.Row="0">
            </Grid>
            <ItemsControl Width="Auto" Height="Auto" Grid.Row="0" Grid.Column="1" Name="MainRegion" cal:RegionManager.RegionName="MainRegion" Padding="10" Margin="10, 0, 0, 0" Background="#FFFFFFD5" />
        </Grid>
    </Grid>
</Window>

<UserControl Width="Auto" Height="Auto" Background="Red" HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
     <!-- I want this to take the full width and height of the parent -->
</UserControl>

<ItemsControl>
  <ItemsControl.ItemsPanel>
    <ItemsPanelTemplate>
      <Grid/>
    </ItemsPanelTemplate>
  </ItemsControl.ItemsPanel>
</ItemsControl>

<ItemsControl>
  <ItemsControl.ItemsPanel>
    <ItemsPanelTemplate>
      <DockPanel />
    </ItemsPanelTemplate>
  </ItemsControl.ItemsPanel>
</ItemsControl>

<ItemsControl>
  <ItemsControl.ItemsPanel>
    <ItemsPanelTemplate>
      <DockPanel />
    </ItemsPanelTemplate>
  </ItemsControl.ItemsPanel>
  <ItemsControl.ItemContainerStyle>
    <Style>
      <Setter Property="DockPanel.Dock" Value="Top" />
    </Style>
  </ItemsControl.ItemContainerStyle>
</ItemsControl>