The minimum size for an adult ig?
Not sure what you mean by that... but I'll tackle it, anyway.
We are talking about green iguanas, right?
The average adult green iguana will reach 6ft from head to toe. Notice that's an average. Females are usually smaller than males, and since iguanas never stop growing, older animals could grow larger than 6ft. So I guess you could say 6ft was a minumum - although, again, I'd hesitate to use the word "minumum" since they never stop growing. Does that make sense?
If you're thinking you can keep it small by keeping it in a smaller cage, well... you can't. Undersized iguanas are iguanas who are fed an improper diet and kept under improper conditions. A properly cared for ig will reach it's adult potential no matter what size of cage you keep it in. But they require very large cages in order to be happy.
It's good you're doing your research before you buy. Good luck!
Alika