The longest snake in the world is the reticulated python, which can measure up to 30 feet in length, and the heaviest snake is the green anaconda, which can weigh up to 550 pounds. The weight and length of snakes varies widely by species; for example, the Barbados thread snake measures just 4 inches long as an adult.
Although the reticulated python can grow to extreme lengths, specimens of that size are rare. Most grow to be anywhere from 10 to 20 feet in length. They usually weigh from 2.2 pounds to 165.3 pounds. This type of python is not generally considered a danger to humans, as it is a nonvenomous constrictor.
The same is true of the green anaconda. Despite its propensity to be heavy, a 550-pound snake is the exception, not the rule. Most green anacondas weigh in at around 330 pounds or so and can be up to 20 feet in length with an average diameter of about 12 inches.
Prehistoric snakes were much bigger than even the reticulated python. In fact, in recent years, the discovery of a snake fossil has led researchers to estimate that some prehistoric snakes may have measured up to 49 feet in length, which is about the length of a school bus.