No matter how large the pot is, I would say it would be best to plant the sunflower in the ground for best growth.
Find out if the sunflower has been growing outside or inside. If inside, it should first be gradually acclimated to outside conditions. Put it where it gets only a little direct sun in the morning at first, or under a dappled shade of a tree, and gradually move it to where it gets more and more sun, but avoid the harsher mid-day sun at first.
Ultimately, it would be best planted in full sun and a lone sunflower will definitely need support -- unless this is a dwarf sunflower, a 7 foot bamboo pole would be ideal, re-bar rod will work, or get one of those green vinyl covered metal rods if you want something more decorative (but those are not as sturdy so you may need two).
Sunflower needs a lot of water, though it is drought tolerant when it's bigger.
Plant it where it will be convenient to care for it (somewhere that you'll pass by often) in a well-drained location. I don't believe the soil needs to be super-rich -- I would loosen approx. an 18"diameter circle with a garden fork, dig a hole in the middle 1~2" deeper than the container the plant is currently in (I would add a handful of compost at this point or some good dirt from under piles of leaves under the trees or shrubs), fill the hole 1/2 way with water, then pop the plant out of the container and pop it into the hole. If the roots are tightly coiled around the edge of the pot, try to gently loosen them, but don't force it if you can't. Sunflowers can be sensitive to root disturbance.
Fill the hole with the loosened soil, burying the base of the stem about 4~6" deeper than it was in the pot. Make a shallow moat about 6~8" away from the stem to help hold the water, and water the entire planting area.
I would then cover the area with 4 sheets of newspaper, leaving 2" around the stem uncovered, water that down, and mulch with grass clippings mixed with straw or leaves. Water to soak the mulch. Cut an X in the paper through the mulch and pound in the stake just outside of the plant's root zone at least 1' down (a 7' stake should be pounded down to 6' -- you could mark it first with measurements for fun, or if using a tomato stake, screw on a wooden yard stick.
) You don't want to wait until later because then, you'll end up damaging some of the roots in order to position the support close enough to the plant.
If it doesn't rain, water well (the water needs to soak all the way down to where the roots are -- I'm thinking at least 1 qt, maybe 1/2 gal.) day after tomorrow, then after 3 days, then after 7 days. Thereafter, only water if it hasn't rained in a week (i.e. every 7 days). You'll need to give it correspondingly larger amounts of water as it grows.