title: What is the use of PreferredSize widget in flutter?
body: <p>I'm new in flutter, and I see there are few widgets for the layout design such as SizedBox and Container.
  There is one widget which is <strong>PreferredSize</strong> Widget that I don't know and cannot find any example about it. 
  What makes it different from other widgets such as container and SizedBox which can set height and width?.<br>
  Can someone give an example?</p>