From the Overviews tab in the CA client you can filter the event count by Event Class:
- Start the CA Client
- Select the Overviews tab
- In the bottom left of the window you should see Top Agent Activity (this is the default filter). Click the blue filter icon to the left of the Top Agent Activity bar
- Select Count of Events By | Event Class
This will show you the number of events stored in the database per Event Class. Using the average 8K per event number you can get an approximation of how much space a certain event class is consuming.