Storage capacity directly affects how much data Elasticsearch is able to store. As is the case in many other situations, this is a big and complex requirement to consider, as it depends upon so many other variables that affect the utilization of space.
The primary variable would be the size of the logs/metrics that are sent to Elasticsearch. This depends on the number of logs that are generated daily (or monthly). For example, if the daily log rate is 100 MB, then that means that, to be able to store a month's worth of logs, at least 3 GB of available space is needed (100 MB x 30 days = 3 GB).
Note that this is the minimum space required for a single source. Ideally, some overhead should be accounted for as data changes regularly and a figure of 100 MB/day might not be constant for all of the days in the month or other months might have a higher rate due to higher load. Additionally, once more sources (or clients) are added, data usage will grow accordingly.