Okay lets give you a crash course on clams.. on lighting and ground.
Maxima and Crocea - Normally should be kept under metal halides and minimally HO T5's. They are high light (shallow water) clams, plus they are also rock boring clams. If you perfer to keep them in the sand area, place a rock under them in the sand.
Derasa, Squamosa, and Gigas - These are considered lower light clams, with the gigas still filter feeding when it matures. The problem with these guys is that they grow very quickly.. they do not stay small. I have seen people keep them in Pc's, but to be on a safer side I would put them under a good amount of PC lighting, or better. Remember it is not necessarily the number of watts per gallon, its all about the penetration of light in water for corals and clams. Also the lower light clams are generally sand dwelling clams. If possible keep them there as they are not shallow water clams, and they are not natural rock borers.
Size: Crocea on average gets to be about 6 inches (it is the smallest of the giant clam family)
Maxima: usually reaches around 16 inches
Derasa: usually reaches around 16-18 inches
Squamosa: usually reaches around 16-19 inches
Gigas: usually reaches about 5 feet
Also remember that older clams (typically larger) are hardier than the younger ones, they fair a lot better in aquariums. I do not suggest a clam smaller than 3 inches to a beginner.
I would suggest a derasa as well