It lives only in the rivers and creeks of the Amazon, Orinoco, and La Plata river systems. The giant otter is the largest and the second heaviest (the sea otter is the heaviest) of the Mustelidae or weasel family. Recent news of a rogue otter devouring an alligator in Florida has us wondering: Just how dangerous are these river weasels? They reach adult size at 9 to 10 months old. It is interesting that human/giant otter conflict is emerging as an increasingly common problem. About the Giant Otter This South American otter is the world's largest, at some 6 feet long. Can an Otter Take Down a Human? Another illustration of the size of a Giant Otter: taking into account its much longer tail and shorter legs, it is similar in size to the average German Shepherd dog. Saving the world’s biggest river otter. Photo credit: Patrick Beese, Giant Otter size comparison to human. Males are from 4.9 to 5.6 ft (1.5 to 1.7 m) and females from 3.3 to 4.9 ft (1 to 1.5 m). Males weigh from 57 to 71 lb (26 to 32 kg) and females from 49 to 57 lb (22 to 26 kg) They are cute and fun to watch.