From Carnivora Forum:
Arctodus simus stood as high as two meters (seven feet) at the shoulder on all four legs. When standing bipedally, the animal was over 3.4 meters (11 ft) in total height. It is estimated to have an average weight of 860-940 kg (1900-2500 pounds), around 42% larger than its contemporary the grizzly bear. The largest specimens were found in Alaska and the Yukon Territory. Males were 20% bigger than females. It was the largest land predator during the Ice Age in North America. The skull was unusual due to its lack of a well-defined forehead and the presence of a short broad muzzle, resembling that of Panthera rather than that of any modern bears. The muscles which passed between the broad cheek bones to power the lower jaws were extremely well-developed and are thought to be adapted for bone crushing in order to obtain the rich marrow. An Arctodus lower jaw can be distinguished from those of the genus Ursus by the slanting ridge dividing muscle attachment areas. Rather than having a waddling gait like modern bear species, Arctodus had toes extending straight forward, presumably being able to move with greater ease. In addition, unlike Ursus, Arctodus had a passage on the lower inside portion of the humerus for a slip of muscle (entepicondylar foramen).
Analysis of Arctodus bones showed high concentrations of nitrogen-15, a stable nitrogen isotope accumulated by meat-eaters, with no evidence of ingestion of vegetation. Based upon this evidence Arctodus simus was highly carnivorous, and as an adult would have required over 35 lbs of flesh per day to survive.
The monstrous Arctodus Bear was six feet tall on all fours, yet it could out-run a horse. It was twice the size, and much more powerful than a modern grizzly bear. It’s long legs and ability to reach fast speeds (up to 30 or 40 miles an hour) may have allowed it to run down Pleistocene prey much like a hunting feline. However in this scenario, Arctodus’s size would have been a handicap – it’s skeleton was not built for making the sharp turns characteristic of any predator who hunts agile prey to survive. Rather, scientists think Arctodus moved in a pacing motion like modern bears, with a body built more for endurance than speed. It’s also thought that Arctodus simus used it’s great size and power to intimidate it’s Pleistocene peers like the Dire Wolf and American Lion away from their kills.