What kind of sunflowers? There are many!

Small ones, 2m-high ones. Exclusively yellow ones, ones with having a dark centre, ones with flaming leafs. Thick ones. Thin ones.
I used to have sunflowers until last year on my (not small) balcony, for beside dandelion and cherry blossoms they are my preferred kind of flower. Yes, one year is enough to raise any of them

If it is one of those man-sized plants, just make the hole big enough, and that means: deep. They are deep-rooting, like roses (at least the types I had).