General rule is an enclosure of 18"x18"x30" (45x45x76 cm) for a baby/juvenile and then moved into a bigger cage of around 24"x24"x48" (60x60x120 cm) for an adult. These are the most common pre-made sizes used.
These sizings offer a more vertical style than what you have, but yours will suffice for how old your chameleon is. You may want to upgrade down the road.
