For the larger species, you can use larger systems--preferably tall ones. There isn't necessarily an upper limit for most species as long as the seahorses are able to get the food they need. In the vast majority of situations, training them (hopefully tank-raised individuals, since they are easier in most aspects) to eat out of a small dish is probably the easiest, cleanest way to feed them. It provides a centralized location for any individuals to go ahead and feed to satiation. One exception to this would be dwarf seahorses, which need high enough food density in the water column to feed properly. Those would work better in smaller systems dedicated to their care and unique feeding situation. If you have dwarves in a larger system, it will take substantial amounts of food to reach the appropriate densities. Hope that helps.