here's a javacsript workaround!

Code:
<html>
<head>
<title>Untitled Document</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
</head>
<script>
//set your length here
var max_lentgh = 5;
//holds the value of the text field before it reached max_length
var temp = "";

function doCheck(){
//text = the value of the text field
	var text = document.form1.textfield.value;
	if (text.length < 5){//if length is less than max length store the value in temp
		temp = text;
	}else if(text.length >= 5){//if length greater than max length store put the last value back into the text box
		document.form1.textfield.value =temp;
	}
}
</script>
<body bgcolor="#FFFFFF" text="#000000">
<form name="form1" method="post" action="">
	<textarea name="textfield" cols="10" onkeypress="doCheck()"></textarea>
</form>

</body>
</html>