When I gave it to my friend 2 years ago, it was wedging between the front and back of my 55 gallon tank, so from end to end, it was at least 13" long.
I asked my friend today how the clam was doing and he said it's now 2 cigar ashtrays. It died the other week after having fallen victim to bristle worms. Personally, I think the worms were probably attracted to the clam since it was experiencing a slow decline over the last couple of months, but who knows. The bristle worms were the really small red kind.