Now what do I do??

They'll grow to about 4 or 5 feet tall so a 15" diameter pot should be ok although you'll still need to support them a bit with a bamboo cane and a bit of string. Try and prop you pots up on little pebbles or stones to keep it from being flat on the ground and becoming waterlogged; they don't like wet feet! They need full sun, if possible Phoenix but will tolerate a bit of shade. And they'll turn their faces to follow the sun too!

Thank you all!

According to the packet, they are Gelbe Riesen, Helianthus annus?

Ah, they're going to be patio pot plants then? Again, the size of your pot will depend on which variety of sunflower you've grown. If it's one of the varieties that grown to about 6 feet or so, a pot about 12" or 15" diameter should be big enough. If you've chosen one of the big varieties, like Russian Giant, you'll need the biggest pot you can lay your hands on! As with any plant, the taller it grows, the deeper it's roots will be so that it doesn't topple over. And pot grown plants need to be kept well watered, whatever you're growing. And slugs will still slither up your pots and decimate sunflowers unless you use a deterrent.

Oh I see. Then the bigger the pot the better. You need to put them in their forever pots about 12 ins across unless they are a dwarf variety. You'll need a lot of space and a lot of pots.
Alternatively you could repot into a slightly larger one and then give some of them away.

When you pot them on, hold by a leaf, not the roots.
It's going to be cold over the next few days, I'd leave them until next week before repotting.

Grammaretto they cannot go into the ground, must stay in pots.

So, do I now put them into their forever pots, or just go up a pot size or 2?

I would plant then into the ground now in a sheltered spot pref against a sunny wall about 2 feet apart, and stake them with your canes.
Water well and keep them well watered.
I have a couple to plant out too...

Oops, just saw post!

Any particular size pot?

Oh heck! Sounds a bit high maintenance!

Ok, I currently have 3 small pots, each approximately 3.5 inch diameter, with 3 seedlings in each, well, apart from one which has 2.

The seedlings are getting quite long and leggy, I have some compost and bigger pots, and some canes, so now what?

As PP says, prick out each seedling and put them in individual pots and leave them there until they have a total of 6 leaves. Once they have those, plant them up where you want them to grow. If you're growing one of the giant varieties, such as Russian Giant, you'll need to start staking them because they'll grow at a rapid speed and height. Don't forget to protect them against slugs. Slugs love tender young sunflower plants.

They need to be put into individual pots, then grown on until they are big enough to plant out.