The following Splunk query will return disk space used by the _internal index.

index=_internal source=*license_usage.log type=Usage | eval gb=b/1024/1024/1024 | timechart span=1d sum(gb) as GB by host useother=false | untable _time host gb | top limit=1 host | join time [ search index=_internal source=*license_usage.log type=Usage | eval gb=b/1024/1024/1024 | timechart span=1d sum(gb) as GB by sourcetype useother=false |untable _time sourcteype sum(GB)]