6x6x4 is the minimal acceptable size, which is what makes it 'OK'. Applied to us, it is a comfortable walk-in closet. An extra 2 feet or more in all directions is always more desirable.
Out-door-enclosures, wherever possible, are and always will be the best enclosures for a reptile, if done right. In the long run they are also the most economical (save $ on lighting, heating, and electric bills).
Now, just because everything is all out in the open, it does not mean maintenance, cleanliness, and hygene are lower compared to an indoor. Unless you have a thing for flies, bugs, ants, and termites (the iguana certainly will not), you have to keep it all tidy as you would indoors.
A negative side effect on outdoor'ing an iguana, similarly with monitors etc, is that natural sunlight makes these animals more 'hyper', particularly if they're used to being kept indoors. Also, being outdoors means less human interaction (tho it is what the animal is used to), which means longer time to tame the beast. But, on the plus side, the fresh air, natural sun, and sounds of nature will simulate the natural environment (if there is heavy traffic, or construction in the vicitnity, it may be better in-doors, especially if there is contruction going on).
Currently I do not have an iguana, I used to tho, and I live somewhere in South Florida, between Ft. Lauderdale and West Palm... 


-IJ
p.s. Oh yeah, this brings out another issue, particularly since you're floridian: hurricanes! Make sure that what you build can be easily disassembled and stored, and then reassembled/rebuilt again. This also means, have a place in the house for your iguana ready with heating elements etc. for during/after the storm. If you live in an evacuation zone, plan ahead.