Just to complicate the issue, pythons and anacondas are types of boas; but there are also snakes known simply as boas.
As atrox said, anacondas are by far the heaviest. For any given length, an average anaconda is up to 50% heavier although they can be as much as twice the weight.
The longest snake ever recorded was an Asiatic reticulated python (Python reticulatus) at roughly 33' (close to 10m), but female green anacondas (Eunectes murinus) often get close to 30' with a girth of 44' (1.2m). Their average length, though, is 20-25'. On average, Asiatic pythons grow to 25-28' (8.5m).
There have been uncorroborated reports of both pythons & anacondas 50' in length, but these reports have been largely dismissed by herpetologists as exaggerations.
For instance, in 2003, the Curugsewu Park in Java, Indonesia, displayed what it claimed was the world's longest snake - a 49' reticulated python. The park's snake handler told reporters that the snake had been found on the island of Sumatra.
However, when a British journalist travelled to Java to examine the snake, he measured it at 22'. When he asked the zoo's snake-keeper about the claimed length, he replied, "Look, you must understand that a python's length is not constant."
Just in case your nephew is interested, here are some more snake facts.
The largest venomous snake is the King cobra (Ophiophagus hannah). It can grow to 18' in length.
The shortest venomous snake is the Namaqua dwarf adder (Bitis schneideri) which is only 8" in length.
The fastest snake is the black mamba (Dendroaspis polylepis) which has been recorded at 14mph.
The snake with the longest fangs (2') is the Gabon viper (Bitis gabonica).
The most venomous is the Inland taipan (Oxyuranus microlepidotus) of Australia. 1 drop of its venom contains enough toxin to kill 100 people.
A rattlesnake can still bite up to an hour after death.
The tiny island of Ilha de Queimada Grande, off the South East coast of Brazil, has 4-5 venomous snakes per sq metre. The snakes are a type of fer-de-lance, the Golden Lancehead (Bothrops insularis). The island is so dangerous that the Brazilian navy imposes a ban on travel to it.