I've only got IE9 on my laptop and had to start it up and play with the settings. Like you when I right clicked and went for open in new window I got a half size window but switching tabs of and on made no difference. What I eventually did was resize the new window by grabbing its sides and expanding it until it filled the screen (not maximised). I then closed the original window and then closed the new stretched window. When I now click on open in new window I get a full screen. I haven't re-booted to see if this sticks.