There's always her shoulder. I have been pondering on the big sword thing myself, such as is it really needed? As big swords have become a bit overly common in things but...Eh two good reasons for keeping it would be,
One: it was in her very original design:
http://s37.photobucket.com/albums/e57/k ... tled-3.jpg
and Two: It fits the story
As for how she holds it? ...uhh...perhaps a magnetic ...sword gripper. I haven't figured out every technical aspect of big sword holding in the land of magic and catapults. So forgive me for that XD But if she can hold it, and swing it, and possibly cause 500HP worth of damage on level 5, then I think...uh...it's 4:24AM, I'm guessing whatever I spit out isn't going to be sane. But uh...ya!