Getting a Windows 10 VirtualBox VM to fully utilize the Retina display’s resolution on a macOS host

As of VirtualBox v6 the settings have changed. You can now use the new screen scaling functionality.

For the selected machine: Settings -> Display -> Screen -> Scale Factor, and use the slider/text field to set the desired screen scaling percentage.

In the VirtualBox Manager, navigate to the desired machines settings > Display > HiDPI Support.

Check Use Unscaled HiDPI Output.

In the virtual machine, select your desired resolution.

Make sure you have the Oracle VM Guest Additions installed. Once I installed the latest according to the version of the VirtualBox I installed on my host OS everything worked like a charm.

On your Windows 10 Virtual Machine, at the lower right-corner of the screen you'll see a ^. Click on it, and you'll see the VirtualBox Icon Running. If you see that but you still can get full resolution changes are you have the incorrect version installed.

Open up Windows File Explorer, click on the CD Drive containing the Virtual Machine Addons and install it.

Alternatively you can also hover your mouse towards the top of the screen to expose the VirtualMachine Menu, Select Devices and select Insert Guest Additions CD Image.