Do you mean giant sunflowers or ordinary bedding ones? If you want a huge sunflower you need to prepare a really good planting hole with lots of compost mixed in, and even feed the plant as it is growing, and keep it watered. For ordinary sunflowers, just plant them out after hardening them off and they should grow without problems. Like all annuals and bedding plants, don't be in too much of a hurry to put them out if we are still getting very cold nights as it will just check their growth.
Slugs and snails are also very partial to sunflower seedlings.

ive got 2 boarder varieties in pots (yellow & purple) in my greenhouse, i potted them up about 3 weeks ago, most are 2/3 inches now, it will proberbly be another 2/3 weeks before i plant out.

I have just planted out my giant yellows, they are about 8 inch`s tall, i also have black sunflower on the go, tomorrow in sowing sunflower teddy bear and Vanilla ice variety`s, plenty of feed and support are needed to keep them healthy.