[Xencored]

Try the CSS first

Styles & Templates >> Style manager >> CSS Templates
Editor.css

Look for
".editor_textbox iframe "

Example would be

.editor_textbox textarea,
.editor_textbox iframe {
	width:99%;
	height:200px;
	float:none;
	background-color:{vb:stylevar textedit_background.backgroundColor};
	font:{vb:stylevar content_msg_font};
}

.editor_textbox iframe {
	margin:0;
	padding:0;
	height:206px;
}

Paste me what you have