he has about another inch or so to grow, that will take a very long time in a mantis,
if you want the best setup for him i would say a 20L would be perfect, i say the best would be a 20L with about 2-3" of sand at the bottom a few pieces of LR and have it go to a sump where you can keep the snails are other food he will eat, along with ther filtration
as for food, i have a bage of frozen krill, a bag of frozen silversides, a container of fresh shrimp from the supermarket, she will also get live snails, live shrimp, live fish, and live crabs that would be perfect, i also recommend if you feed frozen to get a vitamin supplement to soak the food in before you feed it to the smithii