This applies to any platform:
To disply full screen mode in Flash or YouTube videos you MUST Disable hardware acceleration!!
1.Make sure you are using the latest version of Flash. To check your version and to upgrade, see Adobe's instructions.
2.Navigate to a page that shows a video that you can't view in full screen.
3.Right-clickHold down the Ctrl key while you click on the video player and click Settings... in the context menu. The Adobe Flash Player Settings screen will open.
4.Click on the icon at the bottom-left of the Adobe Flash Player Settings window to open the Display panel.
5.Remove the check mark from Enable hardware acceleration.