Pfft. 
I explicitly include sensors I’m actually interested in graphing long term data for and my InfluxDB is 2.6GB and growing. I estimate it will top out at about 3 to 4GB as 2 years data retention will be up in another few months but I’ve been adding sensors as I add devices and come up with more things to graph.
This topic details my search for a reliable InfluxDB size sensor, and the hoops I had to jump through to achieve it https://community.home-assistant.io/t/unreliable-influxdb-size-sensor/226871