This was requested a while back, but there has been some interest in it:

<script type="text/javascript">
wFORMS.behaviors.validation.rules.isUnderLimit = { selector: ".validate-length", check: 'isUnderLimit' }
wFORMS.behaviors.validation.messages.isUnderLimit = "This text is too long.";

wFORMS.behaviors.validation.instance.prototype.isUnderLimit = function(element, value) {

var pattern = new RegExp(/validate-length (\d*)/i);
var matches = element.className.match(pattern);

if(matches && matches[1]) {
return (value.length <= matches[1]);
}
return true;
}


base2.DOM.Element.addEventListener(document,'DOMContentLoaded',function(){
document.getElementById("tfa_Description").className = "required validate-length 256";
},false);
</script>

This will limit the text field 'tfa_Description' to a length of 256 characters. By changing the value of the integer constant you can of course change that limit.