How can I resize VirtualBox .vdi disk?
Yes, the following could be used in VirtualBox 6.0.12, as per some blog articles:
/Applications/VirtualBox.app/Contents/MacOS/VBoxManage modifymedium disk ~/VirtualBox\ VMs/NetBSD-8.1-amd64/NetBSD-8.1-amd64.vdi --resize 10240
It changes the size of the disk to about 10GB (from the original 5GB).
For some reason, there's no GUI to do this within the main app itself.