According to Ciochon et al. (1990), Gigantopithecus blacki was 10 feet tall and weighed 1,200 pounds. This is speculative, since it is with some uncertainty that one reconstructs such a massive creature from a few jaw bones and teeth, however many. The way they arrived at this picture was first to estimate the size of the head from the jaw, and then to use a head/body ratio of 1:6.5 in order to determine the body size. For comparison they cite a head/body ratio of 1:8 for the Australopithecus afarensis specimen known as 'Lucy'. The more conservative ratio for Gigantopithecus was arrived at out of consideration of the massive jaw as an adaptation to the mastication of fibrous plant matter (probably bamboo). Gigantopithecus was probably proportionally a markedly big jawed creature. For the head shape they based their assumptions on the orangutan, since evolutionarily they place Gigantopithecus on the same line as the orangutan, finding a common ancestor for them both in Sivapithecus. However, the orangutan could not serve as a model for the body, since it is unlikely that a 1,200 pound ape would be as arboreal. Therefore they chose the largest primates known, the gorilla and the extinct giant baboon Theropithecus oswaldi, as their models for the body. They gave Gigantopithecus an intermembral index 108 (gorilla at 120 + Theropithecus at 95 divide by 2 = 108 rounded up - very scientific!) (Ciochon et al., 1990).
Since Ciochon (et al, 1990) with aid of Bill Munn (Hollywood monster maker/dinosaur reflesher) were interested as well in building a very impressive life size model we would be wise to consider the dimensions with some caution, and note that they represent the biggest Gigantopithecus that could be built rationalized from the actual remains, and that it is a male. Females may have been half the size of the males, since the teeth fall markedly into two distinct size groupings (Ciochon et al., 1990), as I will discuss later in terms of sexual dimorphism and what inferences have been drawn.
Elwyn L. Simons and Peter C. Ettel (1970) paint a somewhat different picture. They trace Gigantopithecus back to a dryopithicine origin and their corresponding reconstruction is essentially a giant gorilla, 9 feet tall, weighing 600 pounds. It is not nearly as attractive as the giant orangutan/gorilla cross created by Ciochon et al. and Bill Munn (1990)