All I want is simply to know how much space my InfluxDB database takes on my HDD. The stats() command gives me dozens of numbers but I don't know which one shows what I want.
Stats output does not contain that information. The size of the directory structure on disk will give that info.
du -sh /var/lib/influxdb/data/<db name>
Where /var/lib/influxdb/data is the data directory defined in influxdb.conf.