I can't view youtube videos in full screen

Well, that might be problem with your browser or something. If you are pressing full screen button then you should get the result.

Try it out with different browser. All try out clear cache and try again.

Few videos uploaded in Youtube are worst in quality. You should opt for other videos. Wait patiently. You will be able to see them in full screen.

there might be an error in your browser just uninstall it or try a new browser.
the problem would be solved

There might be some problem with your browser. Use different browser and find if there is any problem with your browser.

Clear the cache or there might be some problem with your browser change your browser or reinstall it.

There are several screen size options to choose from.

There is an icon on the extreme right down of U tube video that will take you to full screen view and ESC key will take you back to usual view. I find no difficulty in viewing full screen. You need consult a computer software specialist if you face the problem.

When you view a video, you can choose from three different size options: default, Large player, or Full screen. To switch to full screen, click the rightmost rectangular icon in the bottom right corner of the video player. To exit out of full screen, press Esc on your keyboard and if still the problem persists then check your adobe flash player settings.