croceas stay around 6-7", but they would need to be higher in the rockwork most likely, as they're rather light demanding. maxmias can get larger, say closer to 12" or even a bit more, and again these would probably prefer being closer to a 150W MH than the floor of the tank. I'd not rule out derasa or squamosa clams, though. True, they get large, but it takes years and you could then trade them for smaller specimens. A large clam can fetch a good price if you can find a buyer.