How to shrink font size in Textblock to fit width of content AND maintain font aspect ratio

Here's the deal: I have this line of XAML

<Viewbox Canvas.Left="27" Canvas.Top="479" Width="377" Height="21" Stretch="Fill" 
       StretchDirection="DownOnly" VerticalAlignment="Top" HorizontalAlignment="Left">
  <TextBlock Name="TitleText" TextWrapping="Wrap" TextAlignment="Left" FontSize="14" 
           FontStretch="Normal"  FontStyle="Normal" Foreground="White" Width="377" >some long text here
  </TextBlock>
</Viewbox>

I want the font to scale down to fit the contents height and width. What happens now is that the font scales down, but the Viewbox also scales the content horizontally, making the width of the textbox smaller. Here's an example

Example image

If I set Stretch="Fill" the text will fill the widht, but shrinks only font-size in heigth, making the font look horribly ugly Like this.

Is it possible to shrink the text to fit inside a frame so that it uses the whole width and height of the container?

Solution

Continue the work from Marcus:

<Viewbox Canvas.Left="27" Canvas.Top="479" Width="377" Height="21" Stretch="Fill" 
        StretchDirection="DownOnly" VerticalAlignment="Top" HorizontalAlignment="Left">
   <Viewbox Stretch="Fill" Width="Auto" Height="Auto">
       <TextBlock Name="TitleText" TextWrapping="Wrap" TextAlignment="Left" FontSize="14" 
            FontStretch="Normal"  FontStyle="Normal" Foreground="White" Width="377" >some long  text here
       </TextBlock>
   </Viewbox>
 </Viewbox>

I changed it to

Stretch="Uniform"

and

StretchDirection="Both"

and the result seems great, without stretching the word vertically.