The size of a newborn pup has many variables. It all depends on what type of ray you have and how big the mother is. How much you feed the female when she is pregnent and how many pups she has.
If you don't feed the female a lot of food when she is pregnent she will most likely have smaller weaker pups vs a very well fed female.
Also the amount matters, the more pups, the smaller they are to make room. Less pups, they can be of a bigger size.
Also a 26in+ otorongo ray will have bigger pups than a 12in histrix.
So there are a few factors in this.
If you narrow it down to the type of ray, it will be easier to give you an idea.