How do I make a div full screen?

You can use HTML5 Fullscreen API for this (which is the most suitable way i think).

The fullscreen has to be triggered via a user event (click, keypress) otherwise it won’t work.

Here is a button which makes the div fullscreen on click. And in fullscreen mode, the button click will exit fullscreen mode.

$('#toggle_fullscreen').on('click', function(){
  // if already full screen; exit
  // else go fullscreen
  if (document.fullscreenElement) {
    document.exitFullscreen();
  } else {
    $('#container').get(0).requestFullscreen();
  }
});

#container{
  border:1px solid red;
  border-radius: .5em;
  padding:10px;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div id="container">
  <p>
    <a href="#" id="toggle_fullscreen">Toggle Fullscreen</a>
  </p>
  I will be fullscreen, yay!
</div>

Please also note that Fullscreen API for Chrome does not work in non-secure pages. See https://sites.google.com/a/chromium.org/dev/Home/chromium-security/deprecating-powerful-features-on-insecure-origins for more details.

Another thing to note is the :fullscreen CSS selector. You can append this to any css selector so the that the rules will be applied when that element is fullscreen:

#container:fullscreen {
    width: 100vw;
    height: 100vh;
    }