The one I have is 3.5 and the one being delivered any minute now is 2.5-3. So far the past 2 days the derasa clam has had his foot held down on a rock I have in the sand bed. the maxima will probably go higher up. is it a bad idea to put it on a frag rack?
3" and under they are too small to sustain themselves through photosynthesis alone, the mantle just isn't large enough to cover their needs. You're better off target feeding with phyto when they are that small. I would say at 3.5" you can cut down and don't have to do it that often but they always do better when you offer phyto, they are filter feeders.