Agent buffer size is 3GB worth of events that are kept if the agent is running but can't connect with the coordinator.
You can check the number of events waiting under agent status window launched from the agent system tray.
The file where the events are stored can grow up to 3GB and is located:
C:\program files\quest\change auditor\agent\DBScripts\ChangeAuditorAgent.sdf
For the coordinator the receive buffer is set to 1000 events Once the pending event count exceeds that watermark, the coordinator won't accept any new agent event offloads until the buffer has been processed and uploaded to the database. Research and testing was performed, and it was determined that having a Coordinator receive buffer over 1000 results in performance degradation.