Novellas are bigger than a short story, but shorter than a novel.
The length is between 17,500 and 40,000 words. Ideas to keep in mind when writing one...
You may not have enough room to invest in subplots. Make sure the plot is full and, the subplots dove-tail on it if they are included. They can be used to feed conflict into the main plot or as a way to bring your main characters together.
Scenes contain the main conflict and character development--there's no room for lengthy descriptions.
Novels can span a lifetime or centuries. They have hundreds of pages to fill. Novella's have a little more or less than a hundred pages so they typically have a compressed timeline. They answer the question: What's critical right now?
There isn't a lot of time for your readers to jump onto your character's side. Consider writing in first-person or using deep POV if in third-person.