The size of the parents can give you an idea but in no way are the offspring limited to that size.
My 37.5% SD, 31.5% dwarf male is 7' easy at 15 months old. His dad was 6' and mom was 8-9'. Dwarfs still can get to 14' or so. A lot of it will be determined by feeding but also the genetics and how much of what the snake 'uses'. Also i have been told tiger gene makes them bigger too but i have no idea. Caesar is a white albino tiger het snow. Luckily he is just a giant baby and anything so much as a bump on his snoot will make him lay down and sulk like a puppy who did something wrong lol.
Better to plan too big than too small imo. I have Caesar in a T25 which is a 72x30x18 with a 10" deep shelf running across the whole back of the cage. He uses every inch of that cage. 18" would the minimum height i would want to go and for heating, it seems to be the sweet spot. I would also go deeper. I would have preferred a 72x36x18 cage but that extra 6" would have cost me an arm and a leg im sure.