First let me direct you to the FAQ I wrote up:
http://thereeftank.com/forums/showthread.php?t=84096
Tank size past the very small doen't matter that much, except that you need to keep CA/ALK stable, and Clam's suck allot of it! Plus some clams get very large, like Gigas at ~4 feet! Other clams stay smaller, like the Crosea, but it is a very high light clam and requires MH, though Mh would be reccomended for all clams IMHO.
I would say live rock and a skimmer is all you need for filtration, but there are allot of trash skimmers out there, so ask before you buy.
The bigger your tank the better off you are, and always make sure to do your research first, and after your tank cycles, wait a few months before adding your clam so you get a hang of the tank, and the tank gets settled in.
HTH,
Whiskey