So you are looking for pop up window
Use the following code , set these parameters location=0,status=0,resizable=no,scrollbars=0,left=200,top=128, width=400px,height=200px , as per your requirement.
<asp:imagebutton id="Img12" runat="server" imageurl="IMG/ico_16_4212.gif" xmlns:asp="#unknown">
onclientclick="javascript:window.open('http://devntdcrmweb1.dev.wellmark.com/SalesMarketing/main.aspx?etc=4212&extraqs=%3fetc%3d4212%26pagemode%3diframe%26preloadcache%3d1368193680992&pagetype=entityrecord','mywindow','location=0,status=0,resizable=no,scrollbars=0,left=200,top=128, width=400px,height=200px');" /></asp:imagebutton>