Microsoft just released the new Operations Manager 2012 Sizing Helper Tool. There was a similar Tool (basically it is an Excel sheet) for SCOM 2007 R2. But now there is also some guidance on Best Practices Disk Configuration, SQL Temp DB, High Availability and Network Monitoring. This tool can be found here .
If you want to know how much space your OperationsManger database is using you could run a SQL Report. Because it is a feature from SQL Server 2008 R2 it’s easily overseen I just want to show an example.