I have a Windows 10 laptop running Oracle VirtualBox 5.2.22. I have installed Ubuntu 18.04 in a virtual environment and installed Guest Additions (I can copy/paste to/from Ubuntu to/from Windows, so I know it works).
If I increase the resolution from 1280 X 664 on the Ubuntu VM, the desktop goes off the screen (I can no longer see the launcher icon in the lower left corner). I have tried enabling 3D Acceleration, but the screen then becomes too small.
I have a Dell laptop with a resolution of 3840 X 2160, so I should be able to come close to that. It has an Intel HD Graphics 350 card. I have tried the different graphics drivers in the Ubuntu image, but none seem to make a difference.
I installed VB on another (desktop) and created a VM from the same iso. I was able to get a resolution of 2560 X 1495 without any issues. The other machine is an AMD FX-8150, the laptop giving me issues is an Intel I7 (much newer). The desktop has an AMD Radeon R5 video card. I copied that VM folder to my laptop and only got the aforementioned resolution. IMHO, There is something wacky going on with the Intel video drivers.