While in general reptiles do not need as much room as mammals relative to their size iguanas still need what many would. A tank of at least 8 feet high and 12 feet long will do the job. Pin On Reptile Tanks A 20 or 30 gallon tank will be ok for a little while but not very long. . Before your iguana is fully grown a cage 36 inches long 48 inches high and 18 inches deep should be suitable to keep your pet happy. Green Iguanas - 18-30 inches. To save yourself all this trouble if you have adequate space in your home you can spare on room and leave it for the pet. In contrast if you have an adult iguana then you will need more space for it. An iguana needs a warm dry environment with plenty of sun in order to be healthy. For a baby or young iguana up to around 18-inches long a 20-gallon aquarium is adequate. Rhinoceros Iguana Tegu Nile Monitor. How big of a cage do you need for an iguana. An adult iguana howeve...