Hallo Gubi,
das Problem kenn´ ich auch.
Hier ein Vorschlag zu Lösung, der bei mir gefunzt hat:
Maximize Internet Explorer On Opening
Problem: When Internet Explorer opens it can range in size anywhere from that of a postage stamp to almost filling the screen. Clicking the maximize button works for that particular window but the next time you open IE the same situation repeats.
Solution: The default behavior for Internet Explorer is to open at the same size it was at the last time it was closed. While the default does apply in many cases it's not always consistent. Try the following workaround.
Open a single Internet Explorer window to the smaller size.
Drag the corners of the window until it completely fills the screen. Do not use the maximize button to enlarge the window.
Hold down the Ctrl key, keep it depressed, and using the mouse click File and then Exit on the menu bar. Do not use the "X" in the upper right corner to close the window.
Internet Explorer should now open in a full window.
If it still opens to a smaller size repeat the above instructions, substituting the "Shift" key for the "Ctrl" key.