disc-only copies are actually limited to 2GB, so available RAM will place a limit on DB size.
From Programming Erlang,
For massive numbers of entries that you want to be able to
access readily, you might be better off using CouchDB, MySQL, PostgreSQL, or Berkeley
DB, all of which have open source Erlang drivers and APIs available. The upper
limit of a Dets table is 2 GB. This means the upper limit of a Mnesia table is 2 GB if the
storage type is disc-only copies. For other storage types the upper limit depends on the
system architecture. In 32-bit systems the upper limit is 4 GB (4 * 109 bytes), and in
64-bit systems it is 16 exabytes (16 * 1018 bytes).
