Take a look here , too. I did a shoot recently with another videographer who had this rig and it worked very well -- well made and very reasonably priced.
We also used a unit (on the same web site) that measured 12x20.
I rarely do any green screen work however it seems to me that a roll of chroma key green paper, a couple of light stands, a pole and clamps has got to be the way to go. If you need a full body shot you're covered. If the paper gets footprints just roll out some more.
That said I do have a station wagon so transporting a 3M wide roll is no problem.