There is a chapter in Herman Melville's Moby-Dick called "The Town Ho's Story." As even its title suggests, this chapter is a part of the larger book that can easily be detached from the novel as a whole. This chapter, in fact, was several times printed as a short story before the novel was published. It stands up remarkably well on its own, and indeed it is often included in literature anthologies as a "sample" of Moby-Dick. The chapter has many fascinating similarities to and differences from the book and would therefore lend itself extremely well to the kind of project you have been assigned. The icing on the cake is that the chapter and the novel have often been compared and contrasted by Melville scholars, so you would have some interesting secondary sources to consult. Good luck with your project!
You might compare Conan Doyle's "How It Happened" to, say, an Austen novel you've read, perhaps Pride and Prejudice. This comparison would certainly bring out the differences between the two genres. Doyle's story is set in a frame, has a surprise ending, lots of foreshadowing. This would contrast nicely to an Austen novel that has strong character development, setting changes, intricate plotting, and several subplots.
"A Winter's Dream" and The Great Gatsby both by F. Scott Fitzgerald might work nicely -- the short story has almost the exact same premise, but in a much shorter work. I also think that you might be able to find a good pairing in the short stories of Kate Chopin with a comparison to The Awakening. Many of her stories share very similar themes.
I'm still not clear about exactly what you're looking for, but it seems to me there would be some value is studying both a short story and a novel by the same author and making some of the points of comparison you mention from there. Based on the time period on which your course seems to focus, I'm going to suggest the short story "Rappaccini's Daughter" and The Scarlet Letter, both by Nathaniel Hawthorne. I teach them both, and it's an interesting study in genre differences/style similarities no matter which of the two I teach first.
A similar pair I've enjoyed teaching is F. Scott Fitzgerald's The Great Gatsby and "The Sensible Thing." Each novel shares similar themes and other elements with its "companion" short story, which makes them a great jumping off point for the kind of discussion I think you're trying to have. Best of luck!
Are you asking for suggestions about which short stories and novels you could compare for this essay? Well, my recommendation would be that you pick a short story and novel based on a similarity of theme, which would give you lots to write about in your actual essay. Therefore, try and pick two texts that discuss or look at similar issues. One of my favourite short stories is "The Garden Party" by Katherine Mansfield, which presents us with the central character of Laura who undergoes a move towards an awareness of class consciousness through the course of the story. In a sense, what occurs due to the death of a working class man moves her from innocence to experience. This journey from innocence to experience is a theme of many novels, but you might like to think about using a novel such as The House on Mango Street, which charts the growth, maturing and development of Esperanza as she moves from a state of childlike innocence to one of saddened experience. Although these two different texts have very different settings, the universal theme of innocence and experience is what unites them, making them worthy for a comparison essay. I hope this suggestion helps. I have included links below to the themes section of the enotes study guide for both texts. Good luck!
my prof gave us the following: "Novels and short stories are two important subcategories of prose. They hav a lot in common but they also differ in many aspects. As for example length, development of characters, plot, setting, narrator and others. Comment and concentrate on these stylistic devices and use short stories to support your agument!"
Thanks a lot for your answer. That really sounds great. However, we should keep it more general. It is for an essay that will be the final exam at my university. So it should probably have the development of the short story with Poe's definition and an explanation about the difficulties on how to distinguish a short story and other genres - using mainly the novel as a comparison. Everything should be around the short story anyway as the course was called "The American Short Story" and we had an overview over short stories like RIP van Winkle and others.