Maximum size depends on the species, but is typically around 20 cm under aquarium conditions, but can be substantially less if you have Toxotes microlepis rather than Toxotes jaculatrix or Toxotes chatareus. On the other hand, wild Toxotes chatareus are big fish, around 30-40 cm in length when mature.
As for eating other fish, YES, they're highly predatory given the opportunity. It isn't necessary to give them live fish though, and their optimal diet would be mostly insects anyway. If you want to see them hunt, then offer them crickets, flies and the like. Most specimens do perfectly well on quality pellet foods like Hikari Gold.
Cheers, Neale