Your ig isn't fully grown, so a 4 ft X 6 ft floor, with 4 ft high walls, will work fine for now. Iguana easily accept terrestrial setups.
Unfortunately, the ideal size for a grown iguana's cage is 8 ft wide, 7 ft tall, and 5 ft deep. Huge, but not huge to the iguana. For him, it's merely adequate.
This statement was first described by experts like Melissa Kaplan, Jen Swofford, etc. decades ago, and is still the current truth.