Tall sunflowers are not as huge as the species name, H. gigantea, might lead you to believe. The plants are up to three meters tall, but that's not extraordinary for sunflowers. The flowers are a little under three inches across, or less depending on conditions. There are a lot of sunflowers; USDA lists 51 species in North America. Tall sunflowers fit right in with the rest. The ever popular common sunflower is the outlier. It's an annual, most are perennial. And those flowers can be much larger than the others.
Here now is a little technical info on the flowers. When I say flower above, I'm being rather careless. Sunflowers are in the aster family, and what you see in each head is actually a marvelous collection of many flowers. The large yellow rays are each an individual flower. In the center are numerous smaller flowers. In this image you can see the tubular corolla of some of them. The projecting brown parts are the stamens, fused together into a tube. Emerging from the tubes, looking like asterisks, are the pistils. Each flower can create one seed. All in all a very successful strategy, leading to the largest family of flowering plants, and a great deal of enjoyment, sustenance, and other things for humans. For example, the Cherokee used powdered tall sunflower as a sneeze powder.