Well this is yet another confusing result from TV out. I run my monitor at 1280x1024 (VGA connection to 17" Samsung SyncMaster 170MP LCD) and my TV at 800x600 (S-video cable) and I have no problem with the video displaying full screen - no need to fiddle with the mouse.
Now, I have to admit that when I am not playing a video, the TV displays only a portion of the 1280x1024 desktop, since the TV is in 800x600 resolution - but that's fine by me.
Big Ol' ThomCat