The savannah cat is the biggest cat in the world. No breeder can tell you exactly how big your savannah kitten will become. They grow for 3 years. Nutrition is a big part. Breeding with large genes is always kept in mind here. The first generation savannah cats have the highest probability in being closer or exactly the same dimensions as the African Servals.
They are deemed the biggest cat breed as their length of their legs and torso is what give them the large appearance as opposed to weight; as African serval cats while they look like giant cats really are only between 20 and 40 lbs. Males are generally heavier. We can show you your kittens past litter siblings as adults to give you a more educated guess.