If you are using Splunk 6.0, use the new License Usage Report View. If you are using Splunk 5.x, install the latest version of the Splunk on Splunk app and it has the same view for your 5.x environment.
Following queries can be used for what you are looking for
Total indexed volume by index
|eventcount summarize=false report_size=true index=* | fields index count server size_bytes
Indexed volume for any period
index=_internal source=*metrics.log | eval MB=kb/1024 | timechart span=1h sum(MB) as TotalMB by series
Thanks. I dont know if it changed in one of the updates, but there used to be a screen (I think from the manager tab) that showed you all the information for total data indexed for your license.
I also suggest that you install the Splunk on Splunk (SOS) app...