Is there a maximum size to which the Harvest database can grow?
Harvest SCM versions 12.5 and 12.6, all platforms
No, no hard-and-fast limit beyond the capacity of the server Oracle or SQL Server is running on.
One thing to be aware of is if you would like to archive off projects that are retired, there's a command line utility called "hmvproj" that can move those projects to a separate database for storage and reference, freeing up space on your primary database.
More information on the "hmvproj" command can be found in the CA Harvest SCM 12.6 Command Line Reference Guide.