I did a home-made terrarium which measures 4'x2'x18". Allows a nice gradient setup and plenty of room for him. And because it was homemade, it was cheaper than buying an equivalent sized cage.
Some people will say that it is too large. I don't know where people get the idea that enclosures can be too big... wild pythons live in the great outdoors with no boundaries. As long as you provide adequate hides, it can be as large as you want. And when he is in his roaming moods, he'll be all over the enclosure.