“The size of these specimens – the crown of the molar, for instance, measures about an inch across – helped us understand the extraordinary size of the primate" so this tooth is about twice the actual size!
The above are pictures of Gigantopithecus fossils. Gigantopithecus is a genus composed of two species G. blacki and G. giganteus. G. giganteus lived during the Late Miocene/Early Pliocene and G. blacki is known from the Pleistocene and both are generally considered to be the largest primates ever. Incidentally, G. giganteus is considered to be directly ancestral to G. blacki (so they are really a chrono species). They are estimated to have reached 10 feet in height and about 1,200 pounds in weight. Most specimens come from the Siwalik hills in Pakistan and India. Currently it is thought that Gigantopithecus is actually related to the orangutans, via Sivapithecus (a widespread genus that contains all of the material previously reffered to as Ramapithecus - but that's a long story). At any rate, recent research has thrown an interesting wrinkle into the story:
Using a high-precision absolute-dating method (techniques involving electron spin resonance and uranium series), Jack Rink, associate professor of geography and earth sciences at McMaster, has determined that Gigantopithecus blackii, the largest primate that ever lived, roamed southeast Asia for nearly a million years before the species died out 100,000 years ago. This was known as the Pleistocene period, by which time humans had already existed for a million years.
So it's possible that anatomically modern humans would have encountered them (certainly H. erectus would have also).