The player adapts to give you the fullest screen possible, although there is no full-screen button due to how this player works, but you can maximise your browser window on the device you are watching on.
To maximise your browser window and remove the tabs:
On a mac: click the green button in your browser window or go to 'View' - 'Enter Full Screen' (ensure that your toolbar isn't set to always show)
On a PC: press 'F11' and the function key (often 'fn') or make your webpage fullscreen
On a mobile/tablet: tilt your screen to landscape mode and ensure portrait lock isn't on. You can pinch and zoom to maximise further.
On Chrome: Go to the three vertical dots, go down to zoom and there's a box to the right which if you hover over it says 'fullscreen' - click that!
The camera thumbnails at the bottom of the screen can disappear unless you hover your cursor / finger over it - the bottom right of your screen should give you the option to 'pin' or 'unpin' your cameras.