It should be set to 50 in changeset:   6599:39189dd94f2c.

While it would be nice to allow the user to customize this, the current
mechanism for implementing database schema changes doesn't really allow
for that.

But at least local customization of this is now a one line change 8-).

One thing to note if you are testing this is that db version 7 has one more change
coming: adding python FTS support. So upgrading from this version will require 
manual database changes to get postgres FTS.