Other answers have scaled the persons' mass by the cube of their height, and got answers of about 13 ounces. 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 kg/m²), 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. The ellipse's perimeter would be 13 inches, which is quite stout. (6 * 13" is a 78" waist!)
An elliptical cylinder of water with a width of 2.2 inches, a height of 12 inches, and a depth of 1.1 inches would have a mass of 13 ounces, and a BMI of 4 kg/m². The ellipse's perimeter would be 5.3 inches, which is scaled down from a 32 inch waist.