How big would a bird's wings have to be if the bird weighed 150 pounds?

A young boy named Marlon Lowe was picked up by a bird back in 1977. It flew up a couple feet then dropped him. Witnesses say it had a wingspan of 15 feet. Birds of Prey must weigh twice as much as their prey. Marlon weighed about 60 pounds so they estimate the bird to be around100 pounds.