Don't worry too much about length for any Morelia. Brendls' can get on the heavier side but still can do fine in a 6 foot cage their entire life. They can get long as well but they usually stay coiled up and they still end up being much smaller than any red tail boa in terms of weight. My 3 foot JCP used less of her cage space than sub-adult corn snake.
...amazing snakes. You should think about it.