I would say look to Helianthus.

See if Helianthus grosseserratus fits.

H. grosseserratus has much more narrow leaves that are coarsely serrate. See if H. strumosus fits.

I have such a hard time distinguishing between all the helianthus. H. strumosus is the most likely candidate but this one looked a little different than the ones Im familiar with. Never encountered one this size.

The size of a mature plant is not especially relevant. I can see it every day on my property and in my gardens. For the same species and variety the height of the mature plant can vary from just a few centimeters to a close to two meters, depending on the growing conditions.