I am running VirtualBox on Ubuntu 18.10 -- I tried VirtualBox 6.0 and 5.2.22. The guest OS in VirtualBox is Windows 10 professional. My problem is the 'Auto-Resize Guest Display' is grayed out and I cannot set Windows's display resolution to anything higher than 1600x1200. But my host display is 1920x1080, then I ended up getting a Windows with much smaller display size than the physical display.
I checked I installed the extension pack, with the same version of the VirtualBox main package. I also tried enable/disable the 2D/3D acceleration. No help.
I did some searching on the net and also took some suggestions. For example, I've already uninstalled my VirtualBox 6.0 and installed from scratch a 5.2.22. Do you have some idea?