If you find a message stuck in your outbox, and you are unable to delete this message, it could be that the email is too large for the mail server. Server sending limits are currently capped at 25MB. If you try to open it (to remove attachments) or delete it, an error will say that Outlook has begun transmitting the message.
To fix this, you should first try and set Outlook to offline mode using the File > Offline menu. You will need to wait a few minutes (around 5 minutes or so) before trying to delete the message. Alternatively, set Outlook to Offline mode, close and reopen.