I actually am stuffing one this week! According to the directions, it says ten pounds of stuffing. So far I've put two bags of stuffing (I'm not sure of the weight, I buy it by the bag at wal-mart!) into her, and her legs are full enough. I'm pretty sure she's going to take ten bags of stuffing by the time I'm done. That will be about $35 worth of stuffing! Wowzer!
I'll take a look at the weight of the stuffing tonight and get back to you tomorrow. I can say that if you are in doubt, put more in rather than less. I've always been unhappy with my plushes if I scrimp on the stuffing, and with a little bit of time and punching, they settle more than I expect.
(Is the Wysteria the same size as the giant Frilly Frocks? She's the only giant plush I've seen in person, so she's the only one I can compare to.)