totally depends on which clam you choose, and how much space you have to dedicate to it.
as sk8r noted, T. crocea is the smallest (that i'm aware of at 6" shell length) species of Tridanca clam. T. derasa can get quite large (24"), as can T. maxima (12"), and T. squamosa/ningaloo/noae (16").
if you've already got an SPS dominant cube you're likely already set up to supplement your alk/cal/mag. clams can take up a surprising amount once they get settled and start growing.
if you're running MH/LED you're likely fine as far as light goes. if you're running T5, then it's going to be more dependent on the specifics of the fixtures, clam placement, etc...
i keep a T. crocea and T. derasa. the crocea is a light hog, and i haven't been able to give it too much light thus far (running 4 AI Vegas over a 6' tank). both hardy if their conditions are met, and there is no one in the tank to pick on them.
growth rates can vary wildly for these guys. even clams reared from the same spawn in the same tank can exhibit vastly different growth rates, so ballparking just how fast an individual might grow is nearly impossible.