The general rule for sizing a cage is one and a half to two or more times the lizard from nose to base of tail in length, two thirds to one times the lizard in width, and one to one and a half times the lizard in height. When your monitor reaches adult size, it will be necessary to either purchase or build a custom cage. I built a custom cage for Yoshi. It was built out of plywood with plexiglass in the front, vents on the top and sides, lighting and heating fixtures, and a hinged locking door on the top. Whether you decide to build your own or have someone else build it for you, remember to make sure the cage is secure and has a locking door of some sort.
Savannahs are very smart, very strong, and very good at escaping.
Once they figure out how to get out, they will continue to do it over and over until the problem is fixed. Unless you have the room and the time, overly decorated cages can be a pain to keep clean and maintain. They will also probably dig up, trample, or shred any plants or decorations that you put in their cage. There are several types of ground cover that you can use for Savannah Monitor cages. I use artificial turf, and have had good success with it.