There are a zillion cultivars of H. anuus, and they come in all shapes, sizes, colors, forms.

Yours, whatever kind it is, will never get much taller than it is now. It's an annual, and it is a dwarf form at that.

Here's just one online catalog with 35 different Helianthus annuus varieties. You will see some giants as well as itty bitties.

Sunflowers are so easy to grow from seed.

If you plant seeds in next week or so they'll still have time to bloom before frost hits. I just sowed a bunch of seeds Sunday. According to my past records I've sown seed as late as July 31st for a great display of fall blooming sunflowers.

i would not be expecting much with annuals already with spent bloom ..

and i would not expect to pay more than a few cents for them ...

marked down perennials this time of year... would be a bargain ... based on next years expectations ...

also.. you were on the right track ... a plant has to mature to flower ... so once it flowers.. one would expect it to not get significantly taller .... though i am sure there are all kinds of exceptions ...

ken