To display Windows in full screen on VirtualBox, you will first need to install Windows on a virtual machine using VirtualBox. If you are not familiar with this process, I have a detailed post here on how to do it.
Issue with Windows in full screen on VirtualBox
When you use the Windows in full screen on VirtualBox, it is still not full screen (you have black bars on the side). If you change the resolution to have it full screen you will still be missing some part of the screen.
Installing the VirtualBox Guest Additions
To fix the “black bars” issue and the screen being cut off, you need to install the VirtualBox Guest Additions. To do this, move your mouse to the top of the screen to display the menu. Then click on Devices -> Insert Guest Additions CD image. This will load the drivers on the CD drive of your virtual machine and then display a popup. Then click Run -> Yes -> Yes -> Install. This will install all the drivers needed to display Windows in full screen. Then click Install one more time and click Finish to Reboot now.
Full screen resolution adjusted
Once your Windows machine has been restarted, the Windows resolution will be automatically adjusted to your computer resolution (in my case my Mac). It will take a few seconds to have the Windows resolution adjusted automatically. You can check the new resolution by right clicking on your desktop and selecting Display settings and clicking on Advanced display settings at the bottom right: you will see that the new resolution is now selected there. You are now displaying Windows in full screen on VirtualBox!
Switching between Windows in full screen and Mac
You can switch from Windows to Mac and vice versa by swiping left / right on the trackpad. Enjoy!
If you have any questions about how to display Windows in full screen on VirtualBox or anything else, you can contact me here.