As the title implies, it has a 12 quart capacity, which should be more than enough to cook a large chicken, complete with vegetables. You can increase the capacity by fitting a stainless steel extender ring (supplied).
With the extender ring in place, there's enough room to cook a pretty big turkey - say 16 or 17 pounds. However, you would struggle to get the veg in at the same time. I doubt if that would be a problem for most people - but it's worth noting anyway.
Here's some info on Fagor cookware products: