The bigger the better. This is an animal that get very large and needs a lot of room to move around freely. Plus will eat like crazy. They seem to be a more lazy monitor but they are still very large and need there space to move without being constricted. 4x4x8 would be the smallest for a young adult I would say but giving them the most room you can is only going to make a healthier, happier monitor in the long run.