Animal Farm is usually classed as a novella rather than a full-length novel. George Orwell’s 1945 book, published with the subtitle A Fairy Story, runs to roughly 30,000 words across ten short chapters, based on a word count of the complete public-domain text at Project Gutenberg Canada. That places it inside the range commonly used for novellas, about 17,500 to 40,000 words, and well under the 40,000-word mark often treated as the minimum for a novel.
Because Orwell framed it as a fairy story and an animal allegory of the Soviet Union, it is also described as a satirical fable. Publishers and teachers sometimes still call it a short novel, since the line between the two forms is a convention rather than a fixed rule. For more on how the forms differ, see What is the difference between a novel and a novella?
Source: George Orwell, Animal Farm: A Fairy Story (1945), full text via Project Gutenberg Canada.
Image: Farm pigs feeding, by SteveStrummer. CC0, via Wikimedia Commons.