Get DIV width and height in javascript

I'm trying to get the div width and height as the user changes it and submit that number to another page. I can't seem to figure out how to get the width and height though.

<script>
$(function() {
  $( "#set div" ).draggable({ 
    stack: "#set div",
    preventCollision: true,
    containment: $('#main_content'),
      stop: function(event, ui) {
          var mydiv = document.getElementById("set");
          var pos_x = ui.offset.left;
          var pos_y = ui.offset.top;

          var width = mydiv.style.width;        ----THIS DOESN'T WORK
          var height = mydiv.style.height;      ----THIS DOESN'T WORK

          var window_width = window.innerWidth;
          var window_height = window.innerHeight;
          var need = ui.helper.data("need");

          console.log(pos_x);
          console.log(pos_y);
          console.log(width);
          console.log(window_width);
          console.log(need);

          //Do the ajax call to the server
          $.ajax({
              type: "POST",
              url: "updatecoords.php",
              data: { x: pos_x, y: pos_y, need_id: need, width: width, height: height, window_width: window_width, window_height: window_height}
            }).done(function( msg ) {
              alert( "Data Saved: " + msg );
            });  
      }
  });
});
</script>

What's the proper way to do this?

Since you're already using jQuery, you can just do:

var width;

if (need == 1) {
   width = $("#web").width();
} else {
   width = $("#set").width();
}