Please refer below link
And note: Disable kv-store on your forwarders (if it is heavy forwarder) to avoid additional storage impacts.
Yes, it is mentioned as 5GB
Maintain a minimum of 5GB of free hard disk space on any Splunk Enterprise instance, including forwarders, in addition to the space required for any indexes. See Estimate your storage requirements in Capacity Planning for a procedure on how to estimate the space you need. Failure to maintain this level of free space can degrade performance and cause operating system failure and data loss.