Hi,
i'm not sure how much a fully grown rottie weighs,why dont you check out a rottweiler web site,it will give you all the info you need on rotties.
As for royal canin, its the only dry food that my dogs will eat,from what i have seen and heard it is a good food and they do have a very good reputation,i live in Australia and royal c. is the best dry food we have here,in my opinion:) your dog will let you know if it's any good,by his coat condition,firm stools and if he empties his bowl every time etc.
christine