The database sizing guidelines are in the documentation Sizing and scalability requirements for the Database Server - Documentation for BMC TrueSight Capacity Optimization 11.5…
The formula is 50 GB for 1 million rows daily import using the default aging.
Where rows = #Entities * #Metrics * #resolution (hourly is 24)
Example: If you have 12 million rows it is 600 GB of storage. 12 million rows is 5000 Entities * 100 Metrics * 24 (hourly)
The 250 GB case only assumes 5 million rows import at most. (~2000 entities only * 100 * 24)
You can get an idea of how much data is being imported by looking at the data volume reports
Technical support can also assist you in providing the an official sizing based on your BCO requirements and current configuration .
They will ask you for a data flow report. You can open a ticket with support .