I was measuring my roof last night and 48" wide (outside) would put the edges even with the inside of the roof rails in the rear, and 72" long would span from the rear of the sun roof to almost the back hatch (I'd need to see if there would be an issue with opening the gate.. maybe 65-68" would be better.
I wouldn't want my rack to be wider than the I side edge of the roof rails, basically for the sake of mounting options but also because I may put extra accessories on the sides that I would like to stay "above" the roof. Looks like 47-48" is the sweet spot.