Something better would be:
<Grid Width="Your-specified-value" >
<ScrollViewer>
<TextBlock Width="Auto" TextWrapping="Wrap" />
</ScrollViewer>
</Grid>
This makes sure that the text in your textblock does not overflow and overlap the elements below the textblock as may be the case if you do not use the grid. That happened to me when I tried other solutions even though the textblock was already in a grid with other elements. Keep in mind that the width of the textblock should be Auto and you should specify the desired with in the Grid element. I did this in my code and it works beautifully. HTH.