Tridacna gigas clam
The T. gigas clam can grow to be several feet in size. The one in this picture was acquired at 23" wide by a local fish store for their 1000g display. A big tank is required by anyone wanting to care for this species of clam. It's going to need a lot of space, and as you might have surmised will be utilizing calcium constantly. This type of clam belongs to expert reefers, and even those people find them challenging.
Myself, I visit them at the Dallas World Aquarium. They have a display tank that has two or three Gigas in it, and the system is a 1200g system.