Official rules per Wikipedia:
The wife to be carried may be your own, or the neighbor’s, or you may have found her further afield; she must, however, be over 17 years of age.
The minimum weight of the wife to be carried is 49 kg (108 lbs.) If she weighs less than 49 kg, she will be burdened with a rucksack containing additional weight to bring the total load to be carried up to 49 kg.
Stolen from Reddit.