The cultivated sunflower (Helianthus annus var. giganteus) has flower heads 15 to 30 cm across.
Flora of Alberta keys genus Helianthus as having "at least the lower leaves opposite". Yet it keys Helianthus annus as having "leaves mostly alternate". Budd's Flora also states that the leaves of this species are "usually alternate".
Budd's Flora keys genus Helianthus as "plants with entire, undivided leaves". The leaves are, indeed, undivided, but many leaves of Helianthus annus are toothed
Flora of Alberta states that the achenes of genus Helianthus bear a "pappus of small scales or awns, soon falling off".
Budd's Flora keys genus Helianthus as having "achenes without barbed awns" (italics my own).
It also keys genus Helianthus as having "pappus composed of scales, bristles, or barbs"; not "pappus absent, or present merely as a short crown".
I have found that neither flora easily distinguishes between Helianthus and Rudbeckia (Rudbeckia hirta is Black-eyed Susan) in their keys. However, they do note that the receptacles of Rudbeckia are more conic and the leaves always alternate.