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 ...
AFAIR the guest additions's sharing feature doesn't work for the macOS varieties. On the other hand none of the mentioned options is necessary to get file sharing running.
The only thing needed is a proper network connection.
The most simple configuration: using the physical NIC of the host as bridged device in the guest. If the physical NIC is already ...