Database vendors like Microsoft, Oracle, and IBM provide recommended resource configurations for optimal performance of their database servers.
e.g. Microsoft recommends a minimum of 1 GB of RAM and recommends 4 GB for optimal performance for SQL Server. It’s pretty likely that in an enterprise environment, you have a properly sized database server that can accommodate the vCenter database without any major issues.
The following article provides more information about the recommended resources for SQL Server 2012: http://msdn.microsoft.com/en-us/library/ms143506.aspx
If you are using SQL, don't forget to set the max memory:
How to: Set a Fixed Amount of Memory (SQL Server Management Studio)
You would need to prepare the disk space for vCenter, Update Manager, and other if there's any.
You can use below sizing calculator:
VC Database Sizing Calculator - VMware
Sizing Estimator for vSphere Update Manager 5.5
For the SQL maintenance, I have listed some KB links related to this, hope this is useful: Links & KBs related to vCenter SQL Database Maintenance