What size tank they are in is not going to govern what size they will grow to. They can grow to their max size in a 10 gallon tank, just as easily as they could in your 65 gallon tank.
Average size Africans is around 6 inches. One of my fish is about 4 inches fully grown. Then I have another one who is 5 inches ans still growing.
Just because they are in a bigger tank now, does not mean they are going to grow any bigger because of the extra space.
-FHM