I bought one online and it grew about six feet with the 8- to 10-inch flower spikes, which were a beautiful fall yellow, and abundant.
I wasn't anticipating the size, though (at least not its first year planted), and it was of course stuck right in the midst of smaller growing plants. Awkward, but still awe inspiring.
I moved it that fall, and it didn't come back. I hope to get seeds to sprout, though.