Using jQuery:
parent.html
<body><script type="text/javascript" src="https://code.jquery.com/jquery-2.1.4.min.js"></script><style>iframe { width: 100%; border: 1px solid black;}</style><script>function foo(w, h) { $("iframe").css({width: w, height: h}); return true; // for debug purposes}</script><iframe src="child.html"></iframe></body>child.html
<body><script type="text/javascript" src="https://code.jquery.com/jquery-2.1.4.min.js"></script><script>$(function() { var w = $("#container").css("width"); var h = $("#container").css("height"); var req = parent.foo(w, h); console.log(req); // for debug purposes});</script><style>body, html { margin: 0;}#container { width: 500px; height: 500px; background-color: red;}</style><div id="container"></div></body>