Have you been able to export any video using vegas, did you have the
same problems with the Task from week 5? I'd suggest opening a new
vegas file and keeping it short and smaller resolution to see if it
will export, just to test whether it is a problem with Vegas itself or
that particular vegas file. 1280x720 is a standard size that should be
as big as you need go, I think Russell uses 720x480 in his Vegas
Tutorial video.
You should make it smaller regardless of whether it is the problem in
Vegas because it will take a while to upload it to youtube otherwise.