Given this XAML, I need to resize the userControl in response to user input. How can I set a new width for ListBoxItem (or perhaps the StackPanel)?

                   <ScrollViewer x:Name="ScrollViewer" Margin="0" BorderBrush="Transparent" Width="165"
                  VerticalScrollBarVisibility="Auto" HorizontalScrollBarVisibility="Hidden">

                <ListBox x:Name="AttributeListBox" ItemsSource="{Binding Attributes}" BorderBrush="Red"  Width="160"
                     Foreground="AntiqueWhite" Background="Transparent" IsEnabled="False" HorizontalAlignment="Stretch">
                    <ListBox.ItemContainerStyle>
                        <Style TargetType="ListBoxItem">
                            <Setter Property="HorizontalAlignment" Value="Stretch"/>
                            <Setter Property="Width" Value="150"/>
                            <Setter Property="Margin" Value="0,-2,0,0"/>
                            <Setter Property="HorizontalContentAlignment" Value="Left" />
                            <Setter Property="Template" Value="{StaticResource ListBoxItemSansFocus}" />
                        </Style>
                    </ListBox.ItemContainerStyle>
                    <ListBox.ItemTemplate>
                        <DataTemplate>
                            <StackPanel x:Name="ListBoxItemStackPanel" HorizontalAlignment="Stretch" 
                                        Orientation="Vertical" >
                                <TextBlock FontSize="10" Text="{Binding Key}" Foreground="White"
                                           FontWeight="Bold"  HorizontalAlignment="Stretch"
                                       Margin="2,0,0,0" TextWrapping="Wrap"/>
                                <TextBlock FontSize="10"  Text="{Binding Value}" Foreground="White" Margin="6,-2,0,0" TextWrapping="Wrap"/>
                            </StackPanel>
                        </DataTemplate>
                    </ListBox.ItemTemplate>
                </ListBox>
                </ScrollViewer>