Hi You Can use Javascript to achieve this.
Add following JavaScript function to yoyur page
function ShowTextLength()
{
var objLbl = document.getElementById("lblTextLength");
var objTxt = document.getElementById("txtBox");
if(objLbl && objLbl)
objLbl.innerText = objTxt.value.length;
}
Call this function on body Load and key up event of text box,
<body önload="ShowTextLength();"></body>
Add Text Box and Labels on the page
<asp:textbox id="txtBox" runat="server" onkeyup="ShowTextLength();">
<br />
<asp:label id="lblTextLength" runat="server">
Here I have used Label to display length of the text present in text box, you can take any control and modify last line of the javascript function as per control.