My machines are having vmdk as disk, so I had to convert first the vmdk to vdi
VBoxManage clonehd "source.vmdk" "cloned.vdi" --format vdi VBoxManage modifyhd "cloned.vdi" --resize 16000
After that I've just used the vdi file for my new machine. Ok if the size was increased ( logically) lets do it also on the guest OS. For that I've tried the Gpart Live CD but it didn't work so I had to download the Knoppix LiveCD boot from it in the VM, delete the swap, extend the partition and then adding the swap partition again and of course with the help of the GPart.