My guys was just in small spots that started of about the size of 5 cent coins and got up to about a 10 cent coin before it started healing up. The top of his head was the worse spot and a few on the back legs aswell.
I wouldnt be to fazed about the food side of things just yet....seems a bit early for that, but you never know. My guys do well on Blackhawk but some others have had issues.
Certainly somthing to mention to the vet though. My vet has a special diet written up for food sensitive dogs that he recommends people try if the drugs dont work.