Rottweilers are very fast to develop but slow to mature, Belin Von Iron Hearts recommends that they should not be considered adult until (AT LEAST 24 months to 3 years of age.) Some of the biggest (especially the males) may be anywhere up to 3 1/2 to 4 years of age before they're finished growing! Never be tempted to try to make your puppy grow faster, or bigger, than comes naturally. This can lead to all sorts of problems including bone/joint conditions and heart problems. It's totally normal for an adolescent pup (anywhere between 12 weeks to 2 years ) to look a bit 'rangy or scrawny' or out-of-proportion.
A pup gains his full height quite a bit earlier some by 6 to 9 months than his full weight or muscle development. As long as he's eating a premium puppy food, and getting enough exercise and proper veterinary care, then he will grow to fulfill his potential at around 3 years of age.
Remember the height, weight, bone size, coloring and so on are all determined by genetics and the biggest indicator of how big your pup will grow to be, and what he will look like in terms of conformation are his parents. He will likely grow be somewhere between them in terms of size/weight - with male pups tending to be closer to their dad's size, and most females to their mom's.