From Citizendium, the Citizens' Compendium
Giant snakes include those specimens large enough to swallow an adult human, and are rare, worldwide. This definition is not a systematic biological classification, of course, but one that has an authentic place in the legal codes concerning captive animals, since giant snakes pose a risk to safety that smaller snakes, of the same species, do not. Giant snakes are of interest to travelers, both reptile enthusiasts, who sometimes seek to find them, and tourists, who are generally eager to avoid them. Both goals are practical, as there are only restricterd places where giant snakes exist in the wild.
What characteristic do giant snakes share? Snakes that fall under this category are usually at least 20 feet long, but also are of considerable girth and weight, so that not every very long snake is considered a "giant". Instead, giant snakes are the oldest individuals of certain constrictor species, such as pythons, anacondas and boas, that are capable of growing to great size. Because giant snakes do pose a danger to humans there are regulations and guidelines for keeping such animals in captivity 
Snakes that are long and heavy enough to take human sized prey are not only limited to a few known species, but must have a habitat with a climate and prey supply that enables a long life with resources to support body growth. Most of these animals, in the wild, are said to be found near large bodies of water. An aquatic habitat for hunting may be required because, when the buoyancy of that water supports their body weight the very large and heavy snake is not handicapped by its size. On land, these snakes have limited speed and mobility as compared to their smaller counterparts.
Reports of snakes weighing tons and spanning lengths of 50 to nearly 100 feet can be found in writings from explorers and adventurers of the tropical world in the Nineteenth and first part of the Twentieth century, but these stories may be no more than tales. There is no doubt, however, that the green anaconda (Eunectes murinus) can grow to 30 feet in length (9 meters) and 1200 pounds in weight (550 kilograms). "Jesús Rivas, along with a team of revolving graduate students and volunteers, has captured and released about 800 green anacondas which live in the Venezuela llanos ecosystem." ( National Geographic News )
Tales of Giant Snakes: A Historical Natural History of Anacondas and Pythons by John C. Murphy and Robert W. Henderson, Krieger publishing, 1997 ISBN 0-89464-995-7
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