The bellow will resize the hard disk up to 20GB (20 * 1024MB).
Clone and convert disk:
VBoxManage clonehd "source.vmdk" "cloned.vdi" --format vdi
Resize disk:
VBoxManage modifyhd "cloned.vdi" --resize 20480
Clone and convert back to vmdk:
VBoxManage clonehd "cloned.vdi" "resized.vmdk" --format vmdk