I ran into some trouble with my rancheros vms and the disk space they had allocated to them. This is how I solved that problem.
Delete all snapshots of the machine.
Increase the disk size.
Take a snapshot in case something goes wrong.
Connect via remote console and mount a gparted cd.
Expand the partition to fill the available space.
Reboot and make sure you update the config with:
rancher: resize_device: /dev/sda
to avoid needing to do steps 3-6 again.
Once you’ve done all that, you should be fine.