To resize a VirtualBox disk image (VDI), go to the directory where the .vdi file is located, it's usually in ~/VirtualBox VMs, and run the following command:

VBoxManage modifyhd file.vdi --resize SizeInMB

For example:

VBoxManage modifyhd Ubuntu.vdi --resize 20000

The command will resize VDI to 20 GB, or more precisely 19.53 GB.