title: Sizing elements to percentage of screen width/height
body: <p>Is there a simple (non-LayoutBuilder) way to size an element relative to screen size (width/height)? For example: how do I set the width of a CardView to be 65% of the screen width.</p>
  
  <p>It can't be done inside the <code>build</code> method (obviously) so it would have to be deferred until post build. Is there a preferred place to put logic like this?</p>