<sigh> Firstly please do not use a "generic" subject like "Outlook". This is
a group for support of "Outlook"! So we KNOW your question is probably about
Outlook. Put a précis of your problem in the subject line.
Secondly you omitted a few minor details like Outlook version, type of
account, how you connect to the Internet, what version of Windows, and any
and all relevant data. Remember, no-one can see your computer except
yourself....

If they exceed the per-message size quota for your e-mail provider then
they never will send.  If your e-mail provider allows you to send 10MB
e-mails for a maximum size, you won't be able to send 500MB e-mails.  If
you are sending oversized e-mails that your mail host refuses to accept,
you will have to delete them from your Outbox and find some other means
of delivering that huge content to your recipient.  

- Load Outlook.
- Put Outlook in offline mode (File -> Work Offline: enable).
- Exit Outlook.
- Load Outlook in its safe mode ("outlook.exe /safe").  This ensures no
plug-ins get loaded when Outlook is loaded.  Reloading Outlook in
offline mode means it won't touch the stuck item.  If Outlook ever
touches the item, you cannot delete it.
- Delete the stuck item in the Outbox folder.
- Put Outlook in online mode (File -> Work Offline: disable).
- Exit Outlook.
- Reload Outlook.