There is a requirement to monitor the Exchange server database size and a few performance indicators to ensure Exchange server is running smoothly.
This could be realized using Performance Monitor. One specific counter you could use is called: “Total Size of Recoverable Items”.
Also, you can monitor the following counter:
MSExchangeIS\RPC Requests It should be below 30 at all times.
MSExchangeIS\RPC Averaged Latency It should be below 50 ms at all times.
For more information about Microsoft Exchange Server Performance, please refer to the following articles:
Troubleshooting Microsoft Exchange Server Performance
For database size, the physical size of the database is the total size of both .edb file and .stm file. There is no fixed recommended size for database. But, it is recommended by Microsoft that a database occupies at most 50% of the logical hard disk space on which it resides, so you could monitor the disk space.