--SOLUTION 1--
<Viewbox x:Name="questionViewBox" Stretch="Uniform" Grid.Row="13" Grid.RowSpan="3" Grid.ColumnSpan="8" Grid.Column="4" Panel.ZIndex="1" StretchDirection="DownOnly">
            <TextBlock x:Name="questionBox" Text="{Binding Question}" VerticalAlignment="Center" FontSize="{Binding FontSizeQuestion}"/>
</Viewbox>

in the logic:
if (model.Question.Length > 15)
    model.FontSizeQuestion = 8;
else
    model.FontSizeQuestion = 15;

--SOLUTION2--
questionViewBox.MaxWidth = questionBox.ActualWidth * 2d;
questionViewBox.MaxHeight = questionBox.ActualHeight * 2d;

--SOLUTION3--
The extension methods but could not get it to work:
public static class ViewboxExtensions
{
    public static readonly DependencyProperty MaxZoomFactorProperty =
        DependencyProperty.Register("MaxZoomFactor", typeof(double), typeof(ViewboxExtensions), new PropertyMetadata(1.0, OnMaxZoomFactorChanged));

    private static void OnMaxZoomFactorChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        var viewbox = d as Viewbox;
        if (viewbox == null)
            return;
        viewbox.Loaded += OnLoaded;
    }

    private static void OnLoaded(object sender, RoutedEventArgs e)
    {
        var viewbox = sender as Viewbox;
        var child = viewbox?.Child as FrameworkElement;
        if (child == null)
            return;

        child.SizeChanged += (o, args) => CalculateMaxSize(viewbox);
        CalculateMaxSize(viewbox);
    }

    private static void CalculateMaxSize(Viewbox viewbox)
    {
        var child = viewbox.Child as FrameworkElement;
        if (child == null)
            return;
        viewbox.MaxWidth = child.ActualWidth * GetMaxZoomFactor(viewbox);
        viewbox.MaxHeight = child.ActualHeight * GetMaxZoomFactor(viewbox);
    }

    public static void SetMaxZoomFactor(DependencyObject d, double value)
    {
        d.SetValue(MaxZoomFactorProperty, value);
    }

    public static double GetMaxZoomFactor(DependencyObject d)
    {
        return (double)d.GetValue(MaxZoomFactorProperty);
    }
}