Biggest Dwarf Lionfish i have seen is about 3 inches...In my opinion Lionfish are not very territorial as .long as they are given adequate room. I would say for a small group of Dwarfs you should have at least a 40 gallon...medium sized lions like radiatas or Antennas a 60-75 Gallon and The big Volitans and Russells at least a 100 gallon.
At the same time if you only want lion the size of the tank can be smaller or if it is a juvenile.