Answer:
Well, as the common voodoo doll is quite small (I sound like an expert or something), the effects must be amplified to the effect that the person linked to the doll is many times larger. For example, moving the doll’s arm involves moving a very small weight around, but when that effect transitions over to the human, there is a much more massive object being moved.
Under that pretense, I think it is safe to assume that the weight of you sitting on your voodoo doll would apply a force to the doll. Let's say 5 newtons (obviously this isn't anywhere near correct). The scale factor would be based on mass, and the average doll weighs 0.3 pounds, based on the Monster High dolls, which were the first ones that seemed of a similar size to voodoo dolls that I could find stats for.
