VBoxManage clonehd "source.vmdk" "cloned.vdi" --format vdi
VBoxManage modifyhd "cloned.vdi" --resize 51200
VBoxManage clonehd "cloned.vdi" "resized.vmdk" --format vmdk
The above will resize the hard disk up to 50GB (50 * 1024MB).
You will need need to go into Windows Disk Management and extend the drive