12 ounces of real high explosive, like Semtex, would do the job if properly placed. Al Quaida has long been figuring the minimum amount of explosive that could bring down an airplane, including an "experiment" in which a Japanese tourist was blown up in his seat with a bomb made of (probably) acetone and hydrogen peroxide, with a timer assembled from a watch. I think the shoe bomber was packing something like 4-6 ounces per shoe.
If she could get into the cockpit she could blow out the front windows and possibly disable the controls entirely. That's a lot of ifs, though. I carry an M203 grenade launcher, which has a 40mm round roughly half the size of a coffee mug. An HE round going off inside the cockpit of an aircraft would kill everyone inside and render that aircraft inoperable.
I don't know much about the structural weak points of a large aircraft, though. If the explosive was shaped correctly it would be more effective, but that would depend on what effect she's trying to cause. If it's simply to make a hole in the side of the plane, 12 oz probably won't be that big a deal. If she intends to kill the pilots, that's more likely to bring down the plane.
It would be difficult to speculum speculate on the amount needed.