If you are wondering how to switch between full-screen mode and scale mode in VirtualBox, you should know what the host key is.
Defined In VirtualBox Settings
The Host key allows you to quickly switch between host and virtual machine, where the host is your actual computer VirtualBox runs from
1. Step The Host key is defined in the Settings of your VirtualBox
2. Step Open VirtualBox
3. Step Click on File – Preferences, alternatively hit CTRL + G
4. Step Go to the entry Input and on the right pane you can now change the host key. By default the host key is CTRL-Right
You can use the Host key to switch between real PC and virtual machine.
If you hit Host key + Home (Pos1 on EU keyboard) you can switch between Scale-mode and Full-screen mode. Also handy to know!