It's hard to say how long the word's longest boa constrictor is or was. There's no authoritative source on the subject, and credible animal-related websites vary. Still, one estimate, 18 feet, appears to be the maximum reported length.
A Constricting Name
Native to Central and South America, boa constrictors are great swimmers and often traverse water, but they generally prefer to stay on land. While some refer to "land" boa constrictors, the adjective in question is superfluous. Most boa constrictors live in hollow logs and mammal burrows, especially at the edges of rainforests and rainforest clearings. They're large snakes -- some can grow to at least 13 feet (4 meters) and weigh more than 100 pound -- although they're much smaller than their contemporary anaconda and python brethren.
Calling a boa constrictor "the world's longest" requires an appeal to other sources who've researched the topic. Ultimately, the information you quote is only as authoritative as whoever vetted it. Having said that. ... The largest boa constrictor ever found, according to National Geographic, was 18 feet (5.5 meters). Rainforest Adventures Discovery Zoo and Children's Zoo at Celebration Square parrot this factoid, although none of the three attribute a source. According to Animal Diversity Web, the record length of a boa constrictor is slightly over 13 feet (4 meters). That fact is repeated and cited by the Smithsonian National Zoological Park, though just as a general size guideline, not a record.
In general, female boa constrictors are larger than males. Because of this sexual dimorphism -- that is, differences between the sexes -- it's possible the world's longest boa constrictor is female. The tails of males, however, are proportionally longer than their female counterparts. (Yes, indeed, snakes have tails.) Although they're big relative to many snakes, boa constrictors are dwarfed by several related species. Even in Florida, where non-native boa constrictors have become "a species of special concern" for the USGS, it's not a giant snake. That title is reserved for the anaconda and three species of pythons.
All in the Family
The reticulated python, a relative of the boa constrictor, claimed the 2013 Guinness World Record for longest snake in captivity. As of September 2012, the 8-year-old snake, named Medusa, measured 25 feet 2 inches (7.67 meters). She took the title from a 24-foot long (7.32 meters long) snake, a reticulated python named Fluffy. If you open the contest to extinct snakes, a 40-foot (12.19 meters) snake that weighed an estimated one-and-a-half tons takes the proverbial cake. That's Titanoboa, whose name, if you didn't guess, is a nod to its likely ancestral role as a relative of the modern boa constrictor, according to the Smithsonian Channel. It was more than double the longest extant boa constrictor.