Is there a tool, or a good method to get a rough number for license size? I've got several windows, linux and mac servers running all kinds of apps. Really don't want to do a manual check of event logs and what not if it can be avoided.
there's official documentation on how to estimate your index size requirements here:
http://docs.splunk.com/Documentation/Splunk/5.0.1/Installation/Estimateyourstoragerequirements
This talks about storage size though, not how to estimate license usage.
The best way to determine license usage in my mind is to just do it - set up a splunk instance and index all you need. After a week, check how much it consumed.
Alternatively, check how large your archived log files are. There is no magic formula based on the number of applications or whatever because individual environments are, well, individual.
I was looking for this same info today, and I found this doc which gives a few commands which will give the info I think you're looking for:
http://wiki.splunk.com/Community:TroubleshootingIndexedDataVolume