VirtualBox Display Settings

Settings - Display

Screen - Graphics Controller

VBoxVGA

This emulates a graphics adapter specific to VirtualBox. This is the default for images created for versions of VirtualBox <6.0.0 and for Windows guests before Windows 7. Using it on a Linux guest requires installing the guest additions because this adapter is not (yet) supported by the mainline Linux kernel.Only supports OpenGL 1.1 on 64bit Windows 10 and all Linux guests.

VMSVGA

This emulates the VMware Workstation graphics adapter with the "VMware SVGA 3D" acceleration method.Currently the default for Linux guests. This is supported by the mainline Linux kernel using the SVGA driver. Supports OpenGL 2.1 on all Windows and Linux guests.

VBoxSVGA

This provides a hybrid device that works like VMSVGA (including its new 3D acceleration capabilities), but reports the same old PCI VID:PID as VBoxVGA.This is the default for Windows guests.
For full details see: https://superuser.com/questions/1403123/what-are-differences-between-vboxvga-vmsvga-and-vboxsvga-in-virtualbox

Display Resolutions

If you want to run your VM in full screen mode but none of the display resolutions matches your monitor...

The following instructions tested with an Ubuntu 18.0.4 Guest...
Host
Guest

With the Guest up and running, and with VirtualBox Guest Additions properly installed, run this on the Host...

VBoxManage setextradata global GUI/MaxGuestResolution any

VBoxManage setextradata "myVMname" "CustomVideoMode1" "1920x1080x24"

VBoxManage controlvm "myVMname" setvideomodehint 1920 1080 24

where myVMname is the name of the VirtualBox virtual machine, 1920 is the desired display width (in pixels), 1080 is the desired display height (in pixels) and 24 is the number of bits per pixel. Refer to the Wikipedia link provided for help identifying the BPP required.