Created your div as editable..
<div class="textarea-Simple" style=" line-height:1; margin: 10px 0 0 0;resize:vertical;border: 1px solid #CDCDCD;" id="txtAreaNew" runat="server" contenteditable="true" data-maxlength="300" data-minlength="50"></div>
<span id="error">Min 50 character or max 300 character required</span>
now you have to check text count on every key up by below jquery code..
$(document).ready(function() {
$('#txtAreaNew').keyup(function(){
var limit = parseInt($(this).attr('data-maxlength'));
var minlimit = parseInt($(this).attr('data-minlength'));
var text = $(this).html();
var chars = text.length;
if(chars >= limit){
$('#error').show();
var new_text = text.substr(0,limit);
$(this).html(new_text);
}else if(chars < minlimit){
$('#error').show();
}else if(chars == 0){
$('#error').hide();
}else{
$('#error').hide();
}
});
});