What is the earliest opinion on the size of teffilin rosh or yad and what are the differences now?
There is no official minimum or maximum size of tefilin. Just regarding the particularly small ones, you must be especially careful that the writing is kosher and it's harder to get the parchments in and out of them, and regarding the large ones, it is often problematic to keep the head tfilin in the right place, and not have them hanging in the air. We know that the Talmud in Eruvin 95b says that there is room for 2 pair of tfilin on one's head, inferring that their tfilin were relatively small. Similarly the ancient tfilin from 2,000 years ago, found by archeologists in the Judean desert, were very small.Chabad have a custom to buy large tfilin, but as above, there are halachic advantages and disadvantages in doing so.