You can do this in several ways, but I find the easiest
to be :

MainPagesLink.Attributes.Add("onclick", "javascript:window.open('" + "DataUpdates/MainPages.aspx" + "'," +

"'linkwindow', 'width=405,height=225')");

adding an attribute to the button  in your Page_Load or some other page

initialization method. This adds a client-side event handler to the rendered

html, and opens a new window  with DataUpdates/MainPages.aspx in it and a window size

of 'width=405,height=225'.