Dynamically reducing FontSize to avoid overflow

I have written a toy WPF application with a Button and an ItemsControl. Each time you click the Button, the string "AnotherWord" gets added to the ItemsControl. Now, the ItemsControl is displayed as horizontally oriented StackPanel with a fixed width (500 pixels). This means that when you click the button a certain number of times (actually six times), the newly added string gets clipped, like this:

"AnotherWord AnotherWord AnotherWord AnotherWord AnotherWord AnotherWo"

This happens when the FontSize is 13; if you lower it to 12.7 then there's room for the sixth occurence of "AnotherWord". My question is: Is there a way to make this adjustment at runtime so that you avoid the overflow?

EDIT:

In the context of the question, the fixed width of the StackPanel is obligatory - we cannot use more than the 500 pixels we have.

Here is all the code I wrote:

<!-- MainWindow.xaml -->
 <Window x:Class="FontSize.MainWindow"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         Title="MainWindow" Height="350" Width="525"
          DataContext="{Binding RelativeSource={RelativeSource Self}}">
     <Window.Resources>
         <DataTemplate x:Key="labelTemplate">
             <Label FontSize="13" Content="AnotherWord"></Label>
         </DataTemplate>
         <ItemsPanelTemplate x:Key="panelTemplate">
             <StackPanel Orientation="Horizontal" Width="500" Height="50" />
         </ItemsPanelTemplate>
     </Window.Resources>
     <Grid>
         <Grid.RowDefinitions>
             <RowDefinition Height="Auto" />
             <RowDefinition Height="Auto" />
         </Grid.RowDefinitions>
             <ItemsControl Grid.Row="0" ItemsSource="{Binding Path=MyStrings}" ItemTemplate="{StaticResource labelTemplate}" 
                 ItemsPanel="{StaticResource panelTemplate}" />
         <Button Grid.Row="1"  Click="Button_Click"></Button>
     </Grid>
 </Window>
 
 // MainWindow.xaml.cs
 using System.Collections.ObjectModel;
 using System.Windows;
 
 namespace FontSize
 {
     public partial class MainWindow
     {
         public MainWindow()
         {
             InitializeComponent();
             MyStrings = new ObservableCollection<string>();
         }
 
         public ObservableCollection<string> MyStrings
         {
             get { return (ObservableCollection<string>) GetValue(MyStringsProperty); }
             set { SetValue(MyStringsProperty, value); }
         }
 
         private static readonly DependencyProperty MyStringsProperty =
             DependencyProperty.Register("MyStrings", typeof (ObservableCollection<string>), typeof (Window));
 
         private void Button_Click(object sender, RoutedEventArgs e)
         {
             MyStrings.Add("AnotherWord");
         }
     }
 }
 

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