Adding style for asp.net drop-down list control using bootstrap v3
I would say this would be enough:
<asp:DropDownList runat="server" CssClass="form-control" ID="something" />
Take a look at bootstrap docs and scroll down to the Selects section.
You can use this and change btn whatever you want:<asp:DropDownList Width="100%" CssClass="btn btn-default btn-sm" ID="DPCID" runat="server"/>