This, in fact, is the plains bison, which was once found across pretty much the whole of what is now the contiguous United States, leaving aside only the Pacific and Atlantic coasts and the arid deserts of the Southwest; they also lived in far northern Mexico and up as far as central Alberta. The other subspecies, commonly known as the wood bison (Bison bison athabascae) is native to north-western Canada and to Alaska. Physically, it's slightly larger than its southern relative, which means that, in the absence of American rhinos, elephants, and so on, it is, in fact, the largest living, land-dwelling animal of any kind native to the Americas.
Because bison are, indeed, pretty big. A fully grown male can be anything up to 195 cm (6' 6") tall at the shoulder, and can weigh as much as a tonne (2,200 lbs). Females are, admittedly, quite a bit smaller, although at a maximum of about 180 cm (6') and 550 kg (1,200 lbs) they're still pretty big - and closer in size to the males than, say, those of yak. The horns are also about the same size in both sexes, although (relative to the rest of the animal) fairly small in comparison to those of most other bovines.