Can I resize a DOKS node?

DOKS nodes inherit their sizing and capacity properties from their node pool, and you cannot change node pool settings after creation.

To increase the amount of resources your nodes have, you need to create a new node pool of the desired size. After you have created the new node pool and it displays a ‘Ready’ status, delete the older smaller node pool. This causes the workloads on the old node pool to get rescheduled on the new larger nodes.

This does not incur downtime as long as your applications are deployed in a highly available manner with multiple replicas and appropriately sized nodes.

You can review disk usage on your Droplet and then remove unnecessary files.
You cannot resize Droplets to smaller plans, but you can migrate your data to a smaller Droplet.
High RAM or CPU usage is normally the result of applications or kernel processes on the Droplet. You can monitor high CPU usage processes on the Droplet and stop them if necessary.