As pointed out, cages for arboreal snakes should be given more emphasis on height than width. This provides them with the opportunity to climb, as they would in nature and they feel secure on an elevated perch up near the ceiling. That way you will get see them more often as well. If you were to use the dimensions given, you build it: 1800 high x 1000 wide x 600 deep. There are a few different formulae around for calculating cage size for snakes. Probably the commonest is that the width (in this case the height) should be at least ¾ as long as the snake and the depth should at least ½ as long and a minimum of 300 mm height.
I prefer a simpler rule – the length of the two longest dimensions should be roughly equal to or greater than the total length of the snake. So the dimensions above are adequate for an arboreal snake up to a maximum of 3m. Cage dimensions bigger than this are better, if you have the room, but are they are not essential. For example, 3 m pythons can be kept in considerably smaller enclosures - but I believe there is a difference between surviving and living. The other half of the rule than I mentioned above is the relative size of each dimension. The approximate ratio for arboreal snakes is height : width : depth = 4 : 2 : 1. For terrestrial snakes, this approximate ratio becomes width : depth : height = 4 : 2 : 1. Anyway, that's my personal preference.
Obviously if you have a snake that has not yet reached maximum length but is well on the way to, it is prudent to construct an enclosure suitable for housing it when it is at its maximum length. Using the above suggested rule, if it maxes out a little above or below expected, it won't matter.
I hope that is of some help.
Blue