Code to limit the textbox characters

function limitText(objTextBox, maxCharacters)
{
    if(objTextBox.innerText.length >= maxCharacters)
    {
    if((event.keyCode >= 32 && event.keyCode <=126) || (event.keyCode >= 128 && event.keyCode <= 254))
    {
        event.returnValue = false;
    }
    }
}
<asp:TextBox ID="txtDescription" runat="server" TextMode="MultiLine" Rows="6" Columns="80" OnKeyDown="javascript:limitText(this,1024)" ></asp:TextBox>