<script language="javascript" type="text/javascript">
function verifyLength(str, num, checkID){
  var str = document.getElementById(str).value;
  var good = "Your input is long enough";
  var bad = "Your input is too short";

  if(str.length >= num){
    document.getElementById(checkID).innerHTML = good;
    document.getElementById(checkID).className = "verify"
  }
  else{
    document.getElementById(checkID).innerHTML = bad;
    document.getElementById(checkID).className = "verify red"
  }
}
</script>

    <div>
      <label>Password. Minimum 6 characters.<br />
        <input type="password" name="registerFormPassword" id="registerFormPassword" class="field" value="" tabindex="3"
          onkeyup="javascript:verifyLength('registerFormPassword', '6', 'verifyLengthregisterFormPassword');" />
      </label>
      <br />
      <span id="verifyLengthregisterFormPassword" class="verify">&nbsp;</span>
    </div>