Here are the results of our own database size test in the labs, please note that those figures might differ from yours depending on the amount of events, alarms and past performance data available:
100 hosts / 1200 VMs / 100 datastores = approximately 55-60 GBs + the size of the transaction log
10 hosts / 80 VMs / 10 datastores = approximately 6-8 GBs + the size of the transaction log
P.S. if you want us to help you with the database size estimates, feel free to contact our support team for assistance directly.