I have a test system with 150 devices and < 100 alerts and the db is only 22 MB. But the sizing depends on features you leverage. More system updates will take more filesystem. More alerts will take more database.
If you are managing more than 500 servers on a single instance on a WAN of 10 mbps you can reliably do disc, inventory and health monitoring, but patch may be bandwidth challenged.
Even you do a remote db, it should be 8GB/8core etc.
Thanks,
Rob