Change Height and Width of TextArea in codemirror

The CodeMirror user manual is your friend.

Example code:

<textarea id="myText" rows="4" cols="10"></textarea>

<script type="text/javascript" language="javascript">
    var myTextArea = document.getElementById('myText');
    var myCodeMirror = CodeMirror.fromTextArea(myTextArea);
    myCodeMirror.setSize(500, 300);
</script>