Set TextWrapping to True for TextBlock

   
     


<Window x:Class="WpfApplication1.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="LocalInstallFun"
    >
  <TextBlock FontSize="48" TextWrapping="Wrap">this is a test.</TextBlock>
</Window>