Hi ahmadsubuhanlubis,
Please refer sample below.
HTML
<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
<title></title>
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.3/css/select2.min.css" />
<script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.3/js/select2.min.js"></script>
<style type="text/css">
.form-control { width: 200px !important; }
.select2-selection__rendered { font-size: small !important; }
.select2-results__option { font-size: large !important; }
</style>
<script type="text/javascript">
$(function () {
$(".js-example-placeholder-single").select2({
placeholder: "Select",
allowClear: true
});
});
</script>
</head>
<body>
<form id="form1" runat="server">
<div class="container-fluid" style="padding-top: 10px !important;">
<asp:DropDownList ID="ddlsup" runat="server" class="form-control js-example-placeholder-single">
<asp:ListItem Text="-Please Select-" />
<asp:ListItem Text="One" Value="1" />
<asp:ListItem Text="Two" Value="2" />
<asp:ListItem Text="Three" Value="3" />
</asp:DropDownList>
</div>
</form>
</body>
</html>
Screenshot
