A rat pup is equal to an adult mouse so you may want to start with that! At 3 and a half feet long she should be eating small rats. You may want to try 2 rat pups per feeding after you get her switched. Just go with one rat pup per feeding to get her started. You may try keeping the rat in a small container with a mouse so the scent of the mouse gets on the rat. This may help her in eating a rat. Also prescent the room by leaving the rat close to the tank for a couple hours. This will get her really hungry and she should eat right away!