The bigger the better as far as cage size goes. These lizards tend to become overweight and lazy if not given the opportunity to move around and get enough exercise. We built custom cages for our two monitors. They were 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. We use artificial turf, and have had good success with it.