Try this
With only one IE window open, minimize the window. . Then right click the minimized window down at the bottom and select maximize. Close (Big Red X) and reopen the window. Does it now open to the full size?
Couple of things, IE "remembers" the size it was the last time it was closed. Also, certain applications will affect the size of the IE browser the next time you open it. I have a CD with older style games on it like Centipede, Asteroid, Missile Command etc,, and any time I open IE after playing any of these my browser window is set to about 1/4 the full size. Perhaps you have another program interfering with IE. . I know of no way to fix this one except maximizing the window after every time someone plays one of these games.