The scan on startup is enabled by default. So, if you disabled it, you already know how to enable it back 
This is a command line option:
--storage2.piece-scan-on-startup if set to true, all pieces disk usage is recalculated on startup (default true)
or as a parameter storage2.piece-scan-on-startup: true in your config.yaml. If you changed it, you need to save the config (or stop and remove the container in case of docker) and restart the node (or run a new container in case of docker).
But if you didn’t disable it then no changes are required, just restart the node and track the progress of the used-space-filewalker and make sure that it’s not failed.
See