I post these photos a lot, and there are several old threads you can search dealing with caging. I have a male Colombian BCI. This cage is 4 x 2 x 14 inches high. He uses the whole cage and is quite busy on the perches.
If I upgrade the royal python's cage, she will get this cage (above), and the boa will move to a 4 foot X 30 inches deep X 18 inches high cage. It's a bit more depth and height and I can stack the cages if needed.
This is a 40 Gal Critter Cage. It's insulated and rather tall, but IMO even though the current boa cage I have is longer and deeper, I think the royal would be better off in it. These glass tanks just aren't that easy. Top feeding and cleaning is a pain. I had to use Reflectix insulation all over the back, sides, bottom and I made a top cover for it. It looks OK, but plastic caging is the way to go.
You could look at the Constrictors NW web page and look at Pro-Line modular caging.
There is a really nice 5 foot cage with 16 inches of height for $375 including shipping.
If you get the big cage right off the bat, you'll want to look at some type of small plastic tub that you can put in the cage to make a cage within a cage. If you are buying a very young snake that may be the option to use. If you are getting an established feeder that is a little older you may be able to get away with the large cage right away. Ask the breeder what they are doing and follow the advice.