now clay thompson in his article said that it takes 2,700 cu ft of helium to lift 170 pounds. that means it takes about 16 cu feet of helium to lift one pound.
so 4/3 pi r3 is the volume of a sphere so
16 = 4/3 pi r3
(16/4)*3/pi = r3
3.83 = r3
1.56 = r
which means you need a balloon with a radius of 1.56 or roughly 3 feet in diameter to carry up a pound of weight.