Well, in fairy tales, they are usually short. This is because they are often passed down orally and need to be easy to remember. The short length helps in quickly getting to the moral of the story. In detective stories, the length can vary. A short detective story might focus on a single mystery and solve it quickly. However, a longer detective novel can have multiple mysteries, false leads, and more complex character relationships. In science - fiction epics, the length is often long. It's necessary to build the futuristic worlds, explain scientific concepts, and develop the overarching storylines that span across galaxies or different time periods.