Demo

<textarea id="demo"></textarea>
<script>
    $(document).ready(function() {
        $('#demo').textareafullscreen();
    });
</script>

Install

Insert this code before tag </head>

<script src="//ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
<script src="/jquery.textareafullscreen/jquery.textareafullscreen.js"></script>
<link rel="stylesheet" href="/jquery.textareafullscreen/textareafullscreen.css">

Use

$(document).ready(function() {
    $('#demo').textareafullscreen({
        overlay: true, // Overlay
        maxWidth: '80%', // Max width
        maxHeight: '80%' // Max height
    });
});