Hi,
In theory we can address up to 2^64 blocks of 4K each, which comes out to some ridiculous number of exabytes of storage. Of course addressing is only one part of the issue, and that number tells you nothing :-)
For practical purposes the useful size of a store will be much more determined by the amount of RAM available and the speed of random access on the diks (i.e. if you are using spinning platters or an SSD for storage). I have limited resources to do capacity testing, but I have found BrightstarDB to be usable at store sizes in the 100M triples range on a machine with 8GB of RAM and a (last generation) consumer SSD.
Unlike some other triple stores, my goal right now is not to focus on capacity, but to focus instead on usability for developers, so I expect that for the forseeable future a store size of the order of hundreds of millions of triples is going to be where BrightstarDB capacity tops out, but I also expect that if you can afford to throw more RAM at the server you can push beyond that.
Cheers
Kal