Some people get ideas that are focussed enough and short enough to be short stories.

Some people get more complicated ideas that require more words to tell them.

Some people get both, and they only find out when they've actually written the thing how long it needs to be.

Some people write what they think is a short story and find out (from their readers) that it's actually the first chapter in a novel (because the readers want "the rest of the story").

I've been writing (without much luck) for thirty-one years---and, hell, I can't determine my style, or what length I prefer to write in. The last decade I mostly turned out short stories, here and there---then last month something grabbed me and I've shot past thirty-five thousand with the end nowhere in sight. What's my favorite length? Whatever I can persuade somebody to publish (and pay me for)...