It's not based on size. It's usually if they would turn into effective combat forms depending on the levels of calcium. If the Flood is forced to, they can infect Grunts and Jackals and turn them into combat forms. In the main Halo trilogy we only see Brutes, Elites and Humans combat forms while the everything else contributes to Flood biomass which will eventually create a Gravemind to coordinate the Flood and to alter the environment and atmosphere. Don't forget the that the Flood aren't mindless. Once they have a Gravemind they will learn from how the Jedi fight and will eventually learn to overcome them. Plus, all your AI's are belong to us. Logic Plague will get at least any semi sentient AI in the Star Wars universe.