Oops, sorry. Hrm, yeah then you need to worry. That's the second mistake I've made, lol. I misread the package of seeds. Mine are not 5-6 feet. I was reading the package yesterday and it actually said 3.6 meters. So like you they are in the 12 foot range. My nephew will love them...my sister in law is gonna kill me and her husband laughed like crazy when I just called him.
I'm guessing yours will be similar to mine in the flower size. Mine say the flowers get to about 60cm. How old are yours now? I would let them veg until they are at least in the 6 foot range. Now I'm just guessing but weed normally can double in size from veg to flower. I would guess all plants are similar to a degree. I wouldn't think that the flower will start to develop until this time. Once it does, wait a week and then do the switch. Have you read any other gardening sites? if so what do they say? I wouldn't think that a lot of other sites would use DWC to grow sun flowers and this might be the best site. Good luck. Would love to see pictures once they start getting up there.