I have the same problem with a draft with 30mb attached.
try turn the Outlook to Offline and delete the draft but it ask me to tun it on again. if i don't, it just has another option to cancel that task.
so here is the solution, very simple:
1.double click in that draft or email you want to delete (outbox)
2.remove the attachments
3.save it as draft again
4.simply delete it.
5.done

btw i'm using outlook 2011 on Mac OS X