Try this (I found it with Google)
Once you open Outlook, drag the borders of the small window larger, then close the window by double-clicking the icon in the top left of the window. The next time you open a message, Outlook should remember the window size.
Also check if Outlook is set to run minimized:
1. Click on Start > All Program Files > Microsoft Office
2. Locate Microsoft Outlook, right click it and choose Properties.
3. In the open box, click Shortcut tab. After Run, check if minimized if selected. If it is selected, select maximized.
4. Click OK to save the changes.
5. Start Outlook from Start > All Program Files > Microsoft Office > Microsoft Outlook again and verify the status.