<Grid
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
   <Grid.Resources>
    <Style x:Key="Test" TargetType="TextBlock">
      <Setter Property="TextWrapping" Value="Wrap"/>   
      <Setter Property="FontSize" Value="24"/>
    </Style>
  </Grid.Resources>
  <DockPanel>
 
  <Slider Minimum="100" Maximum="1000" Value="300" x:Name="WidthSlider" DockPanel.Dock="Top" Margin="20" />
  <StackPanel  VerticalAlignment="Center" Orientation="Vertical" Width="{Binding ElementName=WidthSlider, Path=Value}" >
    <Border Background="#55FFAA" Padding="5" Margin="0,30">
      <TextBlock Style="{StaticResource Test}" Text="This is a sample sentence that happends to be a bit long but makes sense. Wrapping this should happen either at the beginning of a new phrase or at a space" />
    </Border>
    
     <Border Background="#55FFAA" Padding="5" Margin="0,30">
      <TextBlock Style="{StaticResource Test}" FontFamily="#Kakao Small Sans" Text="이것은 약간 길지만 의미가 있는 샘플 문장입니다. 이것을 래핑하는 것은 새 구문의 시작 부분이나 공백에서 이루어져야 합니다." />
    </Border>
  </StackPanel>
</DockPanel>
</Grid>