I can't answer the question definitively without dragging my bag down from the attic and stuffing it with something. But using the top of my SUV as a template, I estimate the dimensions of the bag (when full) to be 40" x 36" x 18". And that must be a fairly accurate WAG because the volume of the bag using those numbers is exactly 15 cubic feet.
Not sure, but you definitely need luggage racks on your roof to even try. If you do, take the product dimensions and measure your rack. Note that since this is a "bag" and not a hardshell, it is adaptable to fit into a little smaller space if needed.
Yes. We have a Honda CRV with no roof rack. I ran the attached straps through the sunroof and the hatchback door. I purchased the recommended 2 tie down straps and ran them through the rear passenger doors and over the top of the roof bag. Worked like a charm.