that you should perform with Javascript
lets say that you have a DIVwith the name divCount
and that multiline textbox is called txtDescription

on Page_Load event you need to ADD a javascript to the textbox so it will
perform an action everytime the user writes a letter or deletes a letter,
for that you add in the Page_Load this:

txtDescription.Attributes.Add("onkeyup","countLetters();")
txtDescription.Attributes.Add("onkeydown","countLetters();")

in the ASPX page you need to crate the Javascript Function that will count
and tell the Label the exact letter count.

   <script type="text/javascript">
       var min = 10;
       var max = 15;

       function countLetters() {
           var descr = document.getElementById('<%= txtDescription.ClientID
%>');
           var label = document.getElementById('lblCount');

           if ( descr.value.length < min ) {
               label.innerHTML = 'you wrote <strong>' + descr.value.length
+ '</strong> letters so far, you need to write more ' + (min -
descr.value.length ) + ' letters.';
           }
           if ( descr.value.length >= min && descr.value.length < max) {
               label.innerHTML = 'you wrote <strong>' + descr.value.length
+ '</strong> letters so far, you have more ' + (descr.value.length - min) +
' letters than the needed, but feel free to write until ' + max + '
letters.';
           }
           if ( descr.value.length > max ) {
               label.innerHTML = 'the limit is ' + max + ' letters!';
               descr.value = descr.value.substring(0, max);
           }
       }
   </script>

Now, to enforce the user to update more than the minimum characters, all you
need is to validate before update,
let's image that you have a linkButton called lnkUpdate that will fire an
event in the ASPX page to update thoose values in the popup box to the
database:

again in the Page_Load event

lnkUpdate.Attributes.Add("onclick","return validate();")

below the countLetters function, write:

function validate() {
   var descr = document.getElementById('<%= txtDescription.ClientID %>');
   if (descr.value.length < min ) {
       alert('the minimum is ' + min + ' letters, you only wrote ' +
descr.value.length + ' letters');
       return false;
   } else {
       return true;
   }
}

this will send FALSE if the textbox has less chars than the minimum, and
sending FALSE the update link will not fire!

hope this helps.