select Item from Dropdownlist quicly

hi

I used scroll bar fo my dropdownlist

below is code

<asp:DropDownList ID="DDLcity1" runat="server" CssClass="DDlcityE" OnSelectedIndexChanged="DDLcity1_SelectedIndexChanged"
                                                            ForeColor="#ca0202" AutoPostBack="true" onclick="onSelectMouseDown()" onfocusout="this.size=1;">

 <script>
        function onSelectMouseDown() {

            document.getElementById("<%= DDLcity1.ClientID%>").size = 10;

        }
    </script>

problem is :when I select Item from DDL it select Item with delay I want when I select Item from DDL it select it quicly...

Best Regards

Neda