To set the size of the query cache, you need to estimate the number of queries that will be executed frequently against that repository. For example, a query cache will probably not be effective for the profile repository, since the most common type of query is the login query (login=x and password=y), which gets executed once for each login/password combination. That’s why in the profile repository query caching is disabled by default.
If you have no idea how many queries will recur frequently, you can set the size of the query cache to some number like 1000. A query cache of that size can be handled comfortably without running out of memory.
