The Giant Sunflower variety, which grows to 6-7 feet and has a flower the size of a dinner plate, has a rootball the size of a gallon milkjug when it's ripe. You can tell they're ripe when the flower nods down to face the ground and all the petals fall off. Then you cut the flower off the stalk, let the whole thing dry in the sun for a day or two (watch out for birds!), and then rub the seeds out.

I've never tried them in pots, but if you plant them in the ground, space each one 18" apart in full sun.