> I didn't see anything mentioned in the manuals, but is there a way to
> set the size of the db's cache?

Sure, check the firebird.conf, the DefaultDbCachePages parameter.

You can override this setting on per database mode for Classic Server by
specifying the num_buffers connection parameter in JDBC (or was it
set_page_buffers?). For SuperServer the value is on per-server basis.

> Is there also a way to set the amount
> of main memory firebird uses?

That is the cache and TempCacheLimit parameters. Not sure if that can be
overriden via JDBC connection parameters.

> I've been setting the JVM's max heap
> memory size, but it won't prevent firebird from trying to use more
> than that.

If you run Firebird as a server, Java heap won't be used anyway - those
are two different processes. In the embedded mode that does not work
either, since fbembed.dll/.so also use different memory managers. So you
only friend is the firebird.conf

Roman