If you, like me, experienced when you start a RancherOS droplet that the disk size is only 8 gigabytes out of the expected 50 gigabytes, then the following two commands may help:
sudo growpart /dev/vda 1
sudo resize2fs /dev/vda1
Please note that the space between /dev/vda and the 1 in the first command is necessary.
This will resize your active partition to fill up the maximum space and then resize the filesystem to match that space again.
Thanks to DigitalOcean support for providing the above solution.