Wpf Textbox linecount

You can wrap your TextBlock with a Viewbox, like the following:

<Button>
    <Viewbox>
        <TextBlock Text="Text Text" />
    </Viewbox>
</Button>

Edited:

For setting the same fontsize to the whole of the TextBlocks, you can scale the fontsize of the whole of the TextBlocks, according to the TextBlock with the minimal scale.

To achieve the scale effect, the Viewbox wraps the element with a ContainerVisual and, performs a scale on it. You can add an event-handler to the SizeChanged event of the Viewbox. In this event-handler you can use the scale value of the ContainerVisual that wraps the TextBlock to change the fontsize of the other TextBlocks, like the following example:

<Grid>
    <Grid.RowDefinitions>
        <RowDefinition />
        <RowDefinition />
    </Grid.RowDefinitions>
    <Button>
        <Viewbox Name="vb1" StretchDirection="DownOnly" SizeChanged="vb1_SizeChanged">
            <TextBlock Name="tb1" Text="Text Text" FontSize="72" />
        </Viewbox>
    </Button>
    <Button Grid.Row="1">
        <Viewbox Name="vb2" StretchDirection="DownOnly">
            <TextBlock Name="tb2" Text="Text2" FontSize="72" />
        </Viewbox>
    </Button>
</Grid>

private void vb1_SizeChanged(object sender, SizeChangedEventArgs e)
{
    ContainerVisual cv = VisualTreeHelper.GetParent(tb1) as ContainerVisual;
    ScaleTransform st = cv != null ? cv.Transform as ScaleTransform : null;
    if (st != null)
    {
        tb2.FontSize = 72 * st.ScaleX;
    }           
}

In this example we have a Grid that has only 2 buttons and, the 1st TextBlock effects the 2nd. But, you can use the idea also for more complicated cases.