I tried setting the resolution to 1920x1080 using virtualbox preferences, but the setting is ignored as per xrandr.
You could try something analagous to this within the guest: Section "Device"
Identifier "broadcom"
Driver "modesetting"
Option "HDMI-1" "u2720q"
EndSection
Section "Monitor"
Identifier "u2720q"
EndSection
Section "Screen"
Identifier "Default Screen"
Device "broadcom"
Monitor "u2720q"
DefaultDepth 24
SubSection "Display"
Depth 24
Modes "1920x1080" "1280x1024" "1024x768"
EndSubSection
EndSection..which sets the monitor to 1920x1080
Note that I've tried setting 3d acceleration, but this causes the guest to hang as soon as graphics-KERNEL is loaded. Without it llvmpipe software acceleration is used.