@usk, the documentation mentions the worst case disk usage assuming the compaction is disabled. Please note that the actual disk usage may be a lot less. If you are using CE version 4.6 or above, you can enable circular buffer reuse under the “Settings”->“Compaction” and that would help keep the disk usage limited.
The sizing related documentation is being reworked. It may be a good idea to experiment with a subset of your data/workload and find out the actual usage and based on those numbers, arrive at an estimate for the full dataset.