I've never done a Drunkard's Path quilt but I'll give my input to how I would tackle that question. In a 90"x102" quilt there are 9180 square inches. If you want a 115" x 115" quilt, that would be 13,225 square inches. That's about 1 1/2 times larger so if 4 yards of each will do the smaller, 1/2 again would do the larger, 6yards. And to be on the safe side, I always get a little more fabric so 7 yards of each would work. If you have some left over, make pillow cases or add it to your stash. I have a pretty nice stash from using that method.
PS: to figure how much more square inches, I just multiplied the 9180 by 1.25, then 1.5, etc. till I got the approximate number of the larger quilt.