From the host machine:

$> VBoxManage clonehd "source.vmdk" "cloned.vdi" --format vdi
$> VBoxManage modifyhd "cloned.vdi" --resize 51200
$> VBoxManage clonehd "cloned.vdi" "resized.vmdk" --format vmdk

From within the machine:

$> resize2fs -p -F sda1