If your VirtualBox OS is out of space because you make a small vdi image on disk.
You can resize this file and fix the problem.
This can be easily done with the next command.
1 2 3 | C:\Program Files\Oracle\VirtualBox>VBoxManage modifyhd "C:\Users\mythcat\VirtualBox VMs\Fedora26_server_64bits\Fedora.vdi" --resize 11920 0%...10%...20%...30%...40%...50%...60%...70%...80%...90%...100% |