This is one of the more annoying "features" of Internet Explorer.
IE "remembers" the size of the last window which was opened -- every time you close IE.
You can force IE to remember the window sizing that you want with this process:
- Open one (and only one) IE window
- Drag (not maximize) the IE window to the size you want
- Close that IE window
IE will now remember that window size -- until you close IE while a different window size is active.
It's usually just easier to hit the <F11> key to maximize your IE windows.