VBoxManage clonemedium "source.vmdk" "cloned.vdi" --format vdi
VBoxManage modifymedium "cloned.vdi" --resize 51200
VBoxManage clonemedium "cloned.vdi" "resized.vmdk" --format vmdk

To complete things you need to resize the drive too! To achieve this, you might want to download gparted iso and boot from that iso to resize your drive (select the iso from within the virtualbox settings).