If on the small chance you are on a Mac running OS X 10.7 or 10.8, load up UO, load the QuickTime Player, select "New Screen Recording" from the File Menu, click on the record button, drag the mouse over the portion of the screen you want to record, and then click "Start".
When you are done, click on the stop button, let it finish saving the movie from the spool/cache/whatever it uses, and look in your Movies folder (or whatever folder you told it to save screen recordings to). Rename it, go to YouTube, and click on the upload button and select the movie.
The QuickTime Player records in a format that YouTube handles just fine, and it's around 350 MB per 15 minutes of roughly 720p video (1280x720). If you go up to 1920x1080, it jumps up in size.
I've used it for World of Warcraft recordings, but it should handle UO under a VM or Wine just fine. I recommend 720p because it's a decent and standard resolution for videos and it doesn't impact the system much at all when recording and saving. You can go to 1920x1080 or whatever you have the EC set to if you want, but when saving it can take like a minute or longer to save 15 or 20 minutes worth of 1080p video and it can hit the hard drive a bit. With WOW for what I was doing, it wasn't necessary to have anything higher, and I would assume that in UO at 1920x1080 you would have a lot of area around your character.