Of course, the most important thing is how well maintained the tank is, good water quality, sufficient light and stable parameters.
A 30 would be fine, I think, for a few maximas. A gigas or derasa (at full adult size) might be a bit too big (indeed, they might not even fit in that tank). These clams get really huge. However, clams don't get big very quickly, so you can keep anything you want in a 30 for at least a while.
I think maximas don't get much bigger than a foot long, so you might be able to keep one or two adults in a 30 at most.
One problem might be that large clams draw a lot of calcium, and a tank that small might not evaporate enough water for you to get enough kalk in there. If this becomes a problem, you might want to aim a large fan at your sump. This will increase evap so you'll need to use more topoff water (and thus an opportunity for more kalk). If you do this, the temperatur of the tank will drop, so you'll need to adjust your heaters accordingly.