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.