You can enter the fullscreen mode by clicking the fullscreen button from the bottom left corner of the screen.
You can exit the fullscreen mode by clicking 'Esc' from your keyboard.
If you are playing on Facebook Gameroom app, you might need to modify the zoom level from the app as the Gameroom does not automatically go back to default zoom level when entering the fullscreen mode.
If you are only seeing part of the screen at the moment, you'll need to zoom out by clicking the minus sign (possibly multiple times). If you are seeing black bars around the game screen at the moment, you'll need to zoom in by clicking the plus sign.