If you are stuck with an unhealthy telephone-booth style cage, lay it on it's side so your ig can travel around in it. Iguanas do well in terrestrial-style cages. But that would only be temporary. As Brian said, wide (six feet for two-year-olds, 8 ft. for adults), tall (4 to 5 feet for young igs, 7 ft. for adults unless your ceiling will allow a taller cage), and deep (4 ft minimum for adults).
These large adult cages are not so tough to provide when you think about it. They are a one-time investment, and last indefinitely.