Just found the following info on Microsoft Technet:
Most problems with Internet Explorer window size can be shaken loose using a Registry tweak. Launch REGEDIT and navigate to HKEY_ CURRENT_USER\Software\Microsoft \Internet Explorer\Main. In the right-hand pane, locate and delete the value Window_Placement. Deleting this value is harmless, as it will be re-created when needed.
You can also set the size for secondary browser windows--the ones you get when you right-click a link and choose Open in new window. Just open a secondary window, size it as you please, and hold the Shift key down while clicking the window's Close button.