In a discussion on the Engineering Windows 7 blog about moving and resizing windows, Microsoft engineering honcho Steven Sinofsky offers a useful reminder of how to make sure that a pop-up window that's smaller than usual doesn't accidentally become your new default window size the next time IE launches:
When you see the small window open don't close it until you first open up another copy of the application with the "normal" window size. Then close the small one and then the normal one. Of course this is a pain and close to impossible for anyone to find, but likely a better solution than adding a fourth UI affordance on the title bar.
Other solutions, of course, are using a pop-up blocker (or just dumping IE and using Firefox). For mine, I just want Windows 7 to consistently remember standard Explorer windows sizes and settings -- it refuses to let you always launch Explorer maximised, and it never seems to consistently remember the settings you've applied to folders, despite this supposedly being a "feature" of Vista.