Well most suggest a snakes cage be 2/3 the snakes length long and 1/3 the snakes length in width. So a male at the smallest would need a 8ft by 4ft. Most people suggest more room for these large types.

As far as changing the cage size as it grows. I dont know how you could do that in a way that the snake wouldnt break it down. A plastic or pvc cage would be the easiest to clean.