@Bitty1: First of all, please swipe from the top of your screen to the bottom and make sure that 'Auto Rotate' is active the the Quick Settings Menu. I have looked into this for you and can see that it is possible to watch videos in Landscape Mode, however the video itself would need to have been captured in Landscape in order to be viewed in full screen. If it was originally captured in Portrait then you'll get black bars either side of the picture.
