Currently, there are two view modes available in Parallels Desktop for Chromebook Enterprise to run Windows—Window and Full Screen.
By default, Windows opens in the Window view mode.
Enter Full Screen mode
To make it occupy the whole screen, press F3 button at the top of your keyboard.
Exit Full Screen mode
To exit Full Screen, press F3 button at the top of your keyboard.
Note: If you are using your Chromebook as a tablet, your virtual machine will occupy the whole screen by default. In tablet mode, only Full Screen mode is available.
Using multiple Desks
You can put Windows on another Desk and switch between Windows and Chrome OS seamlessly by using a four finger swipe. It applies to both, Window and Full Screen view modes.
Read more about Desks and how to set up them: https://support.google.com/chromebook/answer/9594869?hl=en