Hi
I wouldnt say the bigger the better.
The Very big size clams have some transportation problems, so it wont be wise to buy it just after it arrived from the Ocean.
From my experience, maxima are easier to keep than croceas, but both surely not very easy to keep.
4"-6" Max
3"-4" crocea
are good sizes for those clams.
If it's your very first clam, I'd go for a squamosa or derasa, and see how they're doin in the tank for few months ,before adding a max or crocea.
Ya need a good tank, lots of light as Chris said, feed them if they're small .
hoose the tank mates carefully as some love to munch the clams.