ubuntu as guest in virtualbox at host with high DPI screen
Solution 1:
Finally found a working solution.
Vitualbox -> (select guest) -> Settings -> Display -> Scale Factor
Solution 2:
In Ubuntu 17.10 in VirtualBox. set hiDPI, and scale to 100% in VirtualBox settings. Then open Ubuntu Settings Devices/Displays, Make you window big and you will see a new Gnome setting called scale. Set to 200% and Ubuntu will take full advantage of every pixel in high DPI screen like a mac retina.
See article at: https://teklern.blogspot.com/2017/10/installing-ubuntu-17.html
Solution 3:
Ok, so my temporary solution (until Ubuntu supports high DPI i guess...) was to just go down in resolution on my host system (windows), to half the max resolution (1920x1080).
I don't really want to mark this as solved, since it kinda bypasses the problem in an undesirable manner. (this is just what I'm content with doing.)