The largest them that is commonly found in freshwater is called Spirillum volutans. It is spiral shaped, and can be nearly a millimeter long. Except for Spirillum, it is very difficult to see Monerans except in a compound microscope with special lighting. Animalia or Fungi. Yes - Animalia.
Movement can be by cilia, flagella, or complex, involving parts that contract.
Structure should be complex. Feeding activity may be obvious. No - Fungus. Should be branched, colorless filaments. Usually attached to some piece of decaying matter - may form a fuzzy coating on or around an object.