Use LayoutTransform to transform a TextBlock

   
     

<Page xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  HorizontalAlignment="Center" VerticalAlignment="Center">
<StackPanel>
  <Button>
    <TextBlock>
      <TextBlock.LayoutTransform>
        <ScaleTransform ScaleX="3" ScaleY="3" />
      </TextBlock.LayoutTransform>
      Foo bar
    </TextBlock>
  </Button>
</StackPanel>
</Page>