[wpf / xaml] How do I make the font size increase when a text block is resized

I have a text block in a frame that resizes when the form is resized but I'd like the text font to resize to fill the available area.

I have :-

Code:

---

<Window x:Class="MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Race Timer" Height="380" Width="571" Name="RaceTimerWindow">
    <Grid>
        <Button Content="Start" Height="48" HorizontalAlignment="Left"
                Margin="12,279,0,12" Name="Button_Start"
                VerticalAlignment="Bottom" Width="150" FontSize="20"  />
        <Button Content="Stop" Height="48" HorizontalAlignment="Right" Margin="378,279,12,12"
                Name="Button_Stop" VerticalAlignment="Bottom" Width="150" FontSize="20"  />
        <Grid  HorizontalAlignment="Stretch" Margin="12,12,12,85"
              Name="Grid_TimeClock" VerticalAlignment="Stretch"  Background="Black" >
            <TextBlock HorizontalAlignment="Stretch" Margin="12,6,12,12"  FontFamily="Arial" FontSize="64"
                      Name="ElapsedTimeDisplay" VerticalAlignment="Stretch" Background="Black"
                      Foreground="Gold" TextAlignment="Center"  FontStretch="SemiExpanded">
                <TextBlock.Text >
                    00:00:00
                </TextBlock.Text>
            </TextBlock>
        </Grid>
    </Grid>
</Window>

---

If your ok with the font increasing well then a viewbox is what you want.

http://msdn.microsoft.com/en-us/libr...s.viewbox.aspx