By using
css
.
.ddl{
width: 20%;
height:10%;
}
<asp:dropdownlist id="ddl1" runat="server" cssclass="ddl" xmlns:asp="#unknown">
</asp:dropdownlist>
<asp:dropdownlist id="ddl2" runat="server" cssclass="ddl" xmlns:asp="#unknown">
</asp:dropdownlist>
<asp:dropdownlist id="ddl3" runat="server" cssclass="ddl" xmlns:asp="#unknown">
</asp:dropdownlist>
-KR