[Notes] How to resize a VirtualBox vmdk file
If you want to end having back a vmdk hard disk (maybe you are interested in using the disk in vwmare too), the complete steps are:
VBoxManage clonemedium "source.vmdk" "cloned.vdi" --format vdi VBoxManage modifymedium "cloned.vdi" --resize 51200 VBoxManage clonemedium "cloned.vdi" "resized.vmdk" --format vmdk
The above will resize the hard disk up to 50GB (50 * 1024MB).
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).
P.S. If your new size is too small, you'll get the same error even with your new vdi file.
I was able to resize the vmdk by cloning it and then modifying and resizing.
vboxmanage clonehd "virtualdisk.vmdk" "new-virtualdisk.vdi" --format vdi vboxmanage modifyhd "new-virtualdisk.vdi" --resize 30720