The rack on mine is pretty big. But mine is not supported by the roof, but rather my Safety Devices "ladder rack" with comes directly from the chassis.
https://www.aulro.com/afvb/images/im...016/11/157.jpg
I had the OEM rack but the overall weight you can put up on the roof will limit you more than the rack size. The bigger and heavier the rack, the less actual weight you can put up there. I needed the roof to be re-set ( under warranty) after a trip up to Cape York, the OEM rack as the doors ended up hitting the A-pillar. All we had was a roof top tent and some clothes on top, but would have been right on the limit as far as weight is concerned. The OEM rack was pretty noisy, and I ended up putting in some aluminium inserts in to reduce the noise - the photo shows the plywood trial bits I taped on as a test. My current rack has a no front bar and a mesh insert on the front, and is much quieter.
The OEM rack has 10 feet, and the more of less line up with all the pillar.
https://www.aulro.com/afvb/images/im...014/11/882.jpg
Since I have never driven mine without a roof rack or the roll cage, it is hard to tell about fuel consumption, but I get 10l/100km on average over the past three years.