It depends on how you are going to keep them.How big of a coop do I need for 12 chickens?

If your chickens are going to be completely free range all year round then realistically all you need is perch space and nest box room; say 2 square foot per chicken.
That all goes horribly wrong the minute you need to confine them in their coop, so your climate will dictate coop size to a certain extent.
I there is any reasonable chance that you will need to confine them to their coop for more than a couple of hours then even 4 square foot per fowl is likely to produce chicken version of cabin fever giving mental and physical health issues.