wakesetter - the reason ppl post that they will get too big is because they will. period. They are the reason that this group of clams is called "giant."
..but you'll be able to house it for a nice long time before it gets TOO big. And, yes, under good conditions you can see growth up to 1.5 - 2 inches per MONTH. Make sure you leave room. And it will such up Ca like nothing else..make sure you can supplement it somehow!
John