I agree trochus are the best. Astrea are fine but theyre dead if they fall on their shell.
As to size I think your better off buying snails, shrimp, and fish on the small side. It means you probably don't have any old men in the bunch. Everything dies and the bigger that stuff is the older it is, old stuff (like fish, snails, people and dogs) tend to die more often than young stuff. Which sucks because I am starting to get old, hmm.
Keep in mind too that there are a couple types of Nassarious snails. The big tongas, almost as big as a turbo and the smaller ones, about the size of marble at biggest.