There is a dashboard in Meta Woot! named Meta Woot! License Event Usage, that calculates the amount of license used per event. It does this by cross correlating your license data against event volumes for a given sourcetype. This gives a fairly accurate average license amount per event.
Now if there are specific events identified by an eventtype and they are pretty much the same format/size as most other events in the sourcetype, then the average license amount per event fits fairly well. If the event is maybe double the size of other events, then you could probably just estimate the size.
If you wanted to calculate the amount an event uses in storage (i.e. after compression), you could either just go with an arbitrary 50% compression as a ball park, or if you want accuracy you would need to look at the index sizes, and compare raw size vs disk size to figure out the compression ratio for the index in question, then apply that ratio to your event. A command like dbinspect will allow you to see raw sizes vs size on disk for a given index and/or buckets.
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