In a fresh installation of Ubuntu 20.04 as a guest (Windows 10 as a host) via VirtualBox, the "Auto-resize Guest Display" option is greyed out. Installing (and even re-installing) the VirtualBox Guest Additions did not fix the issue.
Update: Changing the Graphics Controller from VMSVGA to VBoxVGA and disabling 3D acceleration seemed to fix the issue.
Update: Changing the Graphics Controller from VMSVGA to VBoxVGA and disabling 3D acceleration seemed to fix the issue.