Resizing Virtualbox disks

Clones and resizes source.vmdk to resized.vmdk (64GB):

VBoxManage clonehd "source.vmdk" "cloned.vdi" --format vdi
VBoxManage modifyhd "cloned.vdi" --resize 65536
move source.vmdkk source.vmdk.original
VBoxManage clonehd "cloned.vdi" "source.vmdk" --format vmdk
VBoxManage.exe internalcommands sethduuid source.vmdk

Then replace all occurences of the old UUID with the new UUID in your virtual box xml file. The extra space can now be allocated with partitioning tools like gparted, etc.