The width is good. My own GTP is in a 3x2x2. If you go with one that is only 18" high, I would put it on a tall stand or shelf if possible so he is about 5 feet off the floor. They are arboreal after all and will feel more secure if they feel higher up. But I personally do like to give my GTPs more horizontal room than conventionally thought. They thermoregulate better side to side.