This has to be the number one question I get asked as a breeder. While there is no way to promise a size of a rottweiler from a puppy to full grown there is a way to get the “about size.” No I am not trying to tap dance around and cover myself. In the dog world one of the oldest tricks is to take the weight of the sire and dam; then add them together, now divide by two. Something like this (weight of sire + weight of Dam)/2 = the ABOUT size of your puppy at full growth. There is no sure fire way to know for sure; and there is always a give or take depending on the sex of the puppy.
In some cases Rottweilers don’t grow at the time they bred; so the ethical breeder will disclose this at the time of purchasing your puppy but the breeder should still able to give you the about weight based on the Grand Sires / Dams in the pedigree. Sure it might be some work; but here again the ethical breeder should be willing to furnish this information about their bloodlines.