In some cases you will not be able to see the videos in full screen - if you prefer full screen, click YouTube icon
and ask to view the video in YouTube. That will provide the option to
view in a separate window and in full screen mode. These issues are
relevant only to viewing on your computer monitor.
Another option to make the video larger is to hold the Ctrl key and use your mouse wheel to enlarge or decrease the video size.
When viewing on a phone or tablet, the video will be adjusted to allow for smaller screen size