Other answers have scaled the persons' mass by the cube of their height.  This is probably a lower bound; it assumes that a human brain can fit into a space slightly larger than a teaspoonful.

The theory of "Body Mass Index" (BMI) is that people have the longest life when their mass is roughly proportional to the square of their height.  If we start with 6 feet = 180 pounds (a BMI of 24.4), we can extrapolate this to 1 foot = 5 pounds.  This is probably an upper bound; it allows a few cubic inches for the brain.

An elliptical cylinder of water with a width of 5.4 inches, a height of 12 inches, and a depth of 2.7 inches would have a mass of five pounds.