Re: Weird aspect ratio video display
Video playback within a browser typically uses the Adobe Flash plugin. If you only see this issue in fullscreen mode, which is usually a purely software construct that doesn't involve the video driver, it's most likely a bug in the video software itself, i.e. Flash.
Judging by your description, perhaps Flash is getting confused by the information that describes the size/position of you monitors. In Twinview mode, you may try using (or not using) xorg.conf option NoTwinViewXineramaInfo. If you set this option, you should see full-screen mode spread across both your displays, so some of the video will still be cut off. However, if the video is then shown at a size that'd cover both monitors and the dead space (non-visible space below? right of the smaller rightmost monitor), this again probably points at a Flash bug.