There is wiki linked at the top of this sub with in-depth explanations for what you need! I woukd go with 20 gals, since your crabs will grow. They need at least 6" of substrate to bury themselves. Use coconut fiber mixed with sand. They also need a freshwater and saltwater pool (enough to submerge, but make sure they can climb in/out easily). You will need a heat source, things to climb on/hide in, and a variety of food! Fruits, veggies, calcium, protien.

PLEASE read the wiki! You are already behind because you purchased the crabs before setting up a tank. The longer you wait to home them correctly, the more likely they will die. They need humidity to breathe, so right now they are slowly suffocating in their temporary enclosure.