Recently we had a client who experienced a stuck file in the Outbox. It was about 76MB and no way it would send. She also had Outlook 2007 without the ribbon up top. Using the regular "Work Offline" mode didn't free up the file. It still had the file is being sent message. I was able to clear it up by doing the following.
1. Make sure Outlook is in "Work Offline" mode
2. Hit Send/Receive
3. Before the operation finishes, cancel the send/receive in either the popup window for send/receive or the bottom bar.
After I cancelled the send/receive, it made the file free and I was able to delete it.
Hope this helps!