It really depends. Generally, a fantasy fiction novel can be considered long enough if it fully develops the story, characters, and world-building. A minimum of 50,000 words might be a rough starting point, but some can go well over 100,000 words and still be engaging.
It's non - fiction. Ishmael Beah, the author, actually lived through the horrors of war and being a child soldier in Sierra Leone. His account in 'A Long Way Gone' is a memoir, which means it's a true story of his journey from a normal boy to a child soldier and then his struggle to regain his humanity. There are many details in the book that can be cross - referenced with historical events and the situation in Sierra Leone during that time, which further proves its non - fictional nature.