Board Thread:Naruto Discussions/@comment-4682609-20140507083715/@comment-5539377-20140515175003

Elveonora wrote: "I think that Naruto returned the same eye to him, it was said to give back what he had. It cant be the eye from when he was a kid cause it would be too small and would not even fit in his head. I think the Mangekyo eye was returned to him or reformed by Naruto"

The eye's size is fully grown by puberty and doesn't get larger beyond that point I believe.

In any case, he didn't summon Kakashi's childhood eye back. From what I can see, Naruto used Kakashi's genetic material to grow a new eye.