You could try the following suggestions:
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.
OR
To prevent Internet Explorer from continually changing its size every time
it is opened.
This is done by changing the Maximized key to yes.
Start>Run>Regedit
Locate this key:
HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Document Windows.
Now modify the Maximized key to yes.
Note you can save the registry key first by:
On the File menu, click Export.
In the File name box, type SaveKey, and then click Save.
Or create a system restore point prior to editing.
Note: If the registry key is not there, you can simply create one by going
edit > new.
http://support.microsoft.com/default.aspx?scid=kb;EN-US;322756
HOW TO: Back Up, Edit, and Restore the Registry in Windows XP
Disclaimer: Modifying the registry can cause serious problems that may
require you to reinstall your operating system. Use the information provided
at your own risk.