Biggest-est is best-est...
Anyway...
Personally, I'd stay away from the ones that aren't 4ft wide. 4ft fixtures are the "standard" size and anything else costs more because there's less demand for and therefore less production of them. The difference can vary depending on type of fixture, but sometimes a 2ft fixture is literally more expensive than a 4ft fixture, not just more expensive per watt or however else you want to look at it. So yeah, I recommend going with a 4ft chamber, if you have the room. The standard ones most use are the 6'h x 4'l x 18"w ones you can pick up most anywhere that sells the sort of thing. While they ARE metal and they WILL rust, it seems a slower process than one might expect. I've been using one in particular for a few years now with little rust. If you're especially concerned about it, you can spray paint it. But that'll allow you to stick two 2-bulb, 4ft T8 fixtures from most any hardware store over each shelf, which is more than sufficient for the majority of plants (T5s are, of course, better, but they're also significantly more expensive). You can go as cheap as wrapping 3 sides of it up in one or more space blankets (for reflectivity) and a vinyl shower curtain for the front, assuming you want to be able to see into it when it's all closed up. You can also go the more expensive route and use either foil-covered foam boards or foil-covered bubble wrap, both of which you can find in the insulation areas of hardware or home improvement stores.
As far as heat, yes, it will get pretty warm in there if you don't figure out some way for air exchange. What some of us with cooler winters do (myself included, and I know at least swords does the same) is just pump cool air from a window into the chamber as many months as the weather will allow (for me, this is like 10 months out of the year). If ambient temps are cool enough, such as in a basement environment, you can also just pump air in from outside the chamber. If heat turns out to be a REAL big problem for you, you can also relocate the ballasts in the light fixtures - move them outside the chamber, they're the primary source of heat in a T8 fixture. As far as air movement goes, those little clip-on fans would be great for this type of setup.
As mass noted (and this goes for any indoor setup, not just the big metal rack ones), you will indeed want some way to protect your floors from moisture. And if you have the thing pushed right up against a wall, make CERTAIN that no moisture is sneaking out behind the chamber and hanging out between the wall and the chamber, 'cause that is a very quick way to destroy that section of drywall.
Basically, if I were you, I'd build a miniature version of my greenhouse (which can be seen in the build thread in my sig) using just one rack instead of 6. I, personally, feel that's about as sturdy and versatile a setup as a 4' x 18" footprint will give you. I love the flower house concept mksmith posted about, but if I'm imagining it right, you only get one "layer" of plants since if you used some sort of rack, the plants on the top would prevent light penetration to the lower levels. So going 6 feet high with 5 shelves ends up giving you 30 square feet to grow stuff in a 6 square foot footprint.
My $0.02.
