How big is the average size of polygamist family?

It varies very widely and depends on the religion and culture of the family. Some polygamist families are a husband, two wives, and no children, while others may have dozens of wives and a hundred or more children. In the United States, polygamists usually try to stay under the radar since polygamy is illegal. Because of this, most American polygamists only have two or three wives and 10 or so children. Muslim polygamists are not supposed to take more than three wives according to the Qur'an.