Just upgraded to VirtualBox 6 running on a Windows 10 host and swapped to the SVGA driver (which according to the docs is the new default). Guest is Lubuntu 18.04.
I cannot for the life of me make the resolution increase. I've purged and re-installed Guest Additions to no effect. (using VBoxLinuxAdditions.run uninstall, and re-running).
I've also tried the steps listed here: https://www.tecmint.com/install-virtualbox-guest-additions-in-ubuntu/
Rebooted several times between steps, but the resolution still restricted when using Monitor settings. xrandr lists just 3 resolutions, 640x480, 800x600 and 1024x768.
Posted in VBox Forums too: https://forums.virtualbox.org/viewtopic.php?f=3&t=91052