vagrant disk resize

install Vagrant plugin vagrant-disksize

vagrant plugin install vagrant-disksize
If you want to add make sure user has the plugin installed when starting vagrant you can add this in the beginning of Vagrantfile

# Install vagrant-disksize to allow resizing the vagrant box disk.
unless Vagrant.has_plugin?("vagrant-disksize")
    raise  Vagrant::Errors::VagrantError.new, "vagrant-disksize plugin is missing. Please install it using 'vagrant plugin install vagrant-disksize' and rerun 'vagrant up'"
end
Set desired disk size in Vagrantfile

vagrant.configure('2') do |config|
    config.disksize.size = '50GB'
end
Updating existing vagrant box

Do all of the above
Run vagrant halt & vagrant up (You should see something like "Resized disk: old 32768 MB, req 51200 MB, new 51200 MB")
SSH to vagrant box
Run sudo cfdisk /dev/sda
Use arrows to select your disk probably sdaX. Mine was sda3.
Then select resize using arrow keys. Accept the suggested disk size.
Then select write. And answer yes.
You can select quit now.
Run sudo resize2fs -p -F /dev/sdaX You should see something like: "Filesystem at /dev/sda3 is mounted on /; on-line resizing required old_desc_blocks = 4, new_desc_blocks = 6 The filesystem on /dev/sda3 is now 11933952 (4k) blocks long. "
Run df and see that your disk size has increased.