I don't know how much it can hold exactly. I'll have to look at the factory specs and just see how it feels. I will never never never put fuel cans or water cans, RTT or Pelican cases full of firewood or anything like that up there. That type of weight is completely against my belief in vehicle dynamics. I would like to be able to walk on it, but we'll see. I do want to put my 50lb canoe on it, or skiis, a set of bridging ladders, or maybe lightweight luggage. Sleeping bags, clothes, etc.
It's mounted to the roof using the same method as for the genuine accessories extended roof rails. Basically, there is an extra set of mounts built into the roof sub-structure at the back, that are exactly the same as those in the front. They just didn't cut the holes from the factory. It's explained in the Rave CD. You drop the headliner, and find the mounting points. You drill up through the roof panel with a 1/4" bit to locate the holes on the topside. Then you go up top and drill down with a 1 1/4" hole saw to leave a hole in the roof panel and expose the sub-structure. The 6 feet of the rack are 1" aluminum round, drilled and tapped for M8 bolts. The square feet you see on top are just to cover the holes, I'll pretty them up when I'm done, they don't bear load onto the roof panel, and I'll apply silicone to seal.
The dimensions... 46" wide by... IIRC 6-7 feet long.