not sure, i'm growing my first 3. They're all in separate pots about 6in diameter and 6in tall. I'm amazed at how fast they grow and how thick the stem is (one of them is more than 1cm thick). I guess that they would like to be transferred into a bigger pot sometime soon though. ( I don't have any spare soil in my garden, so pots are the only option). As the roots will be pretty dense now, I just make sure that they get enough water and miracle grow. They are growing well but no flower has started yet.
It doesn't answer your question but I thought it would be good to discuss.
Probably get a "hortie" to tell you their secret, Whitehouse, but I do know that there are quite a variety of seeds to buy, because I purchased some of the short/smaller type once.......
Next time you buy a packet of sunflower seeds look out for those which say "Giant Sunflower". There are lots of different varieties of sunflowers. Some are taller than others. Some have different coloured flowers - yellow, orange and even brown. You just have to buy the "Giant Sunflower" variety. Sow the seeds in a pot of compost to begin with to give them a good start. Plant out in really well manured soil when they are a few inches inch and keep them well watered. And once they've flowered, save the black seedheads for the birds to peck - they love them.
oh, and to add onto my post before, I'm unaware what type of seeds mine are, (they were mixed in a bird feed container whihc my mam spilt), and I recognised the little shoots as sunflowers and so decided to put them in a bigger pot and look after them. Mine are approx 18in tall with lots of leaf growth.