You can set TextWrapping="Wrap"
and wrap it in ScrollViewer
with HorizontalScrollBarVisibility="Disabled"
:
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="*" />
</Grid.RowDefinitions>
<TextBlock Text="{Binding Test}" />
<GroupBox Header="Test" Grid.Row="1">
<ScrollViewer HorizontalScrollBarVisibility="Disabled" VerticalScrollBarVisibility="Auto">
<TextBlock TextWrapping="Wrap" Text="{Binding Description}" />
</ScrollViewer>
</GroupBox>
</Grid>