Size and Weight:
About the Cave Bear (Ursus spelaeus):
One of the most famous of all prehistoric mammals--on a par with the Woolly Mammoth and the Saber-Toothed Tiger--the Cave Bear (technically known as Ursus spelaeus) was one of the most common animals of Pleistocene Europe. There are an astonishing number of Cave Bear fossils; some caves in Europe have yielded literally thousands of bones, to the extent that Ursus spelaeus skeletons were processed in bulk for their valuable phosphates during World War I. This doesn't mean, however, that Cave Bears died en masse like herd animals from disease or flash floods; you have to realize that the same caves could be occupied by these beasts for hundreds of thousands of years, so one or two deaths a year added up to a lot of bones! (See also 10 Facts About the Cave Bear.)
There are three popular misconceptions about the Cave Bear. First, Ursus spelaeus wasn't any bigger than modern Brown Bears or Polar Bears; males attained maximum weights of about half a ton, while the much smaller females only tipped the scales at 500 pounds or so. Second, the bulk of the evidence points to Cave Bears being confirmed vegetarians, though they may have supplemented their diets with occasional servings of meat. And third, Cave Bears and early humans didn't overlap in any significant way, though there are some hints that small populations of Neanderthals may have worshiped this intimidating beast; whatever the case, humans certainly didn't hunt the Cave Bear to extinction, as they did so many other megafauna mammals of the last ice age. (However, a new study concludes that early humans may have helped doom Cave Bears by competing with these beasts for warm, dry caves.)
If humans didn't kill off the Cave Bear, what did? Those thousands of Ursus spelaeus fossils have yielded a rich pathology: these bears endured everything from rickets to cancer to kidney stones, and most diseased or undernourished individuals seem to have died during their extended hibernations (some juveniles, too, were probably picked off by roaming predators like Cave Lions; see The Cave Bear vs. the Cave Lion - Who Wins? for an analysis of this bloody scenario). As to the extinction of the Cave Bear, about 25,000 years ago, that was most likely the result of global climate change, with the resulting loss of the vegetation these prehistoric mammals subsisted on.