You could use a javascript function to check the length of it.
An Example:
<html>
<head>
<title>Test</title>
<script language="JavaScript">
<!--
function checkForm()
{
var text = document.forms.testForm.textArea.value
if( text.length > 255 )
{
alert("The string is too long!");
return false;
}
else
{
document.forms.testForm.submit();
}
}
//-->
</script>
</head>
<body>
<form action="jdnsad.html" method="post" name="testForm"
onsubmit="return checkForm();">
<textarea name="textArea" rows="10" cols="30"></textarea>
<p><input type="submit" value="Send" /></p>
</form>
</body>
</html>