This should do it
vagrant box add ubuntu/xenial64
cd ~/.vagrant.d/boxes/ubuntu-VAGRANTSLASH-xenial64/20170223.0.0/virtualbox/
VBoxManage clonehd ubuntu-xenial-16.04-cloudimg.vmdk tmp-disk.vdi --format vdi
VBoxManage modifyhd tmp-disk.vdi --resize 61440
VBoxManage clonehd tmp-disk.vdi resized-disk.vmdk --format vmdk
rm tmp-disk.vdi ubuntu-xenial-16.04-cloudimg.vmdk
mv resized-disk.vmdk ubuntu-xenial-16.04-cloudimg.vmdk