Many times, the question comes up for Microsoft sizing guidelines for Operations Manager/SCOM. The Sizing Calculator XLS is a great resource to use to help answer some of the storage and SQL DB questions as it relates to the various features you enable in your environment.
The sizing calculator takes features beyond windows agents to help size SQL and storage needs, as well as management servers.
The SCOM Sizing Calculator XLS from TechNet helps determine capacity and storage needs for 2012 and 2016. Here is the 2016 System Center SQL matrix
Other SCOM features that change the Operations Manager environment
# of Unix Servers
Application Performance Monitoring (APM)
URL monitoring (transactional and availability)
DB Data retention requirements