Viewing the live stream on a computer
To enter full screen press F11 on PC or cmd and F on Mac; do the same to exit full screen. To enlarge page within browser, press Ctrl and + key. To reduce page, press Ctrl and - key. Use the Cmd and + or - key on a Mac. To restore page size to normal, press Ctrl or Cmd and 0 (zero key). To view full screen place curser in the You Tube viewport and click the box icon in the lower control bar.
If you open the Desktop stream the browser window can be enlarged or reduced by draging a corner. The stream/image within can be enlarged or reduced as already explained using the control or command key. It can then be centred with the lower or right control bars.
Viewing in high definition on smart phone
Open the stream in Chrome browser. Tap on the three verticle dots at top right of screen. Scroll down to 'Request Desktop Site' and click on small box to select the option. You can then choose best quality by tapping on wheel icon in the You Tube lower control bar and setting quality to HD1080
You might also consider projecting stream in high definition from smartphone or computer to a television by either HDMI cable or via WIFI using Chromecast device.