I am running Windows 10 in VirtualBox on my retina display Macbook pro. The maximum resolution I can set in the guest is 1920x1440, but I would like to take advantage of the retina display's 2560x1400 display. I have been scouring the internet to no avail, how do I do this?
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.
Use Unscaled HiDPI Output.
In the virtual machine, select your desired resolution.