When it comes to fiction, stories can usually be classified into four categories (although there's more, filling up the different word count void but the principle remains the same): flash fiction, short stories, novellas, and the novel. What many people assume (including some beginning writers) is that writing one is the same as writing the other. For example, if writing a 10,000-word short story takes you five days, people will assume it will take you twenty-five days to write a 50,000-word novel. Unfortunately, the reality is, writing isn't scientific and reduced to mere numbers. A 50,000-word novel might have the same word count as five 10,000-word short stories, but the skills needed to write a novel isn't identical to the skills needed to write a short story.
That's not to say there's no area where skills overlap, but one can't go writing a novel expecting it's a longer short story and vice versa. That fact is even more pronounced in the extremes (i.e. flash fiction vs novels). I'm not even saying one is more difficult than the other. Each writer has different strengths so one might find writing a novel easier than writing a short story.
When writing a story, what one needs to focus on is the story itself. Story isn't just about word count but various other factors: plot, characterization, pacing, climax, etc. I'm not here to give you the rules (and what I call anti-rules when you're breaking existing rules) of writing. I'm not even a good writer myself and just because I know these facts doesn't empower me to write well--I'm simply aware of them. Executing them is another matter and sometimes, writers simply go with their "gut feel".
But flash fiction, short stories, novellas, and the novel have forms. And there are differences, no matter how minute, that set them apart. Take flash fiction for example. For me it's best described as ultra-compression. There's little time to dally and you have to limit your words to the integral details. And rarely do we see a multiple point-of-view with flash fiction because more often than not, there's not enough words to do justice to everyone. Novels, on the other hand, give you liberties. It's the right place to describe long, exquisite scenes that are there not for the sake of the plot but because it's lovely to talk about. One can also have a huge, epic cast in novels because you have the space to expound on various characters and their habits.
Years ago, I remember meeting with groups interested in publishing and some of them would say "hey, I don't have a short story, but let's publish one chapter of my upcoming novel instead!" This even applies to the comic scene except instead of stand-alone comic in a few pages, instead you get merely a part of a larger narrative. Now what's wrong with that is that well, as I said, short stories aren't novels and vice versa. While some chapters could easily be short stories (and contain all the necessary elements of a good story), not all chapters of a book are good stories by themselves. That's like claiming to eat cake and all you've tasted is the icing. The icing isn't the cake--the cake is the whole package. And because short stories are different from novels, the reader will come out with a different experience.
If you want to use an excerpt from your would-be novel, just say that it's an excerpt. That way, the reader will understand, especially when the story is anything but whole.
A good short story that I recently read was Nikki Alfar's "Beacon" from Philippine Genre Stories Vol. 2. While there's room for expansion, it's really a self-contained story. If Nikki were to expand it to a novella, there's going to be some major rewrites involved, and perhaps even adding new characters. Vin Simbulan's "Wail of the Sun", from Philippine Genre Stories Vol. 1, on other hand, could easily be interpreted as one chapter in a much, much longer narrative. That's not to say that Vin didn't write a great short story--he did. But honestly speaking, Nikki's story was more "self-contained" than Vin's. That's not to say that the former is better than the latter. Merely that they have different forms and Vin's story would probably take the least rewrite if converting it to a novella or a novel (it could also easily be chapter one or chapter ten of a long epic). But that's the nature of short stories too. You could write two similar-themed stories and end up with different products.