Hi, Okay...I measured. I have a 1994 Jeep Cherokee SPORT, not a GRAND Cherokee. Here are the measurements:
Exactly 74 inches long, Exactly 48 inches wide (4 feet)
I do not know the measurements for the mini vans, nor for a Grand Cherokee but I think that they are compatible if not correct. The issue that you need to consider no what you plan on doing is that you'll have to remove the headliner and drill your roof. If you're comfortable doing this....evreything should go ok.
Good luck, Rob.