Create an Ubuntu 18.04 droplet, and use the smallest SSD/HDD size. This way you can make a snapshot of the droplet and have the ability to later deploy that snapshot into a droplet of any size.
In this example, I use the "Optimized Droplet" that is 20GB, since that's the smallest one currently available.
After creating the droplet, power it down and open a ticket with DigitalOcean support, and ask to enable the Ubuntu 18.04 recovery ISO. It can take an hour or two before they do this. By the time you read this guide, hopefully they will have an automated way of doing it.
Once they get back to you, power your droplet back up and choose the recovery ISO's terminal/console option to get a shell. Now shrink the ext4 root to make room for the zfs root:
e2fsck -f /dev/vda1