What you are after is a general impression for your reader. You can't go into too much detail but there needs to be enough for them to pick up and fill in gaps with their imagination.
For example my story Calling the Doctor in From Light to Dark and Back Again is one of my very short tales (100 words or thereabouts) and in a few lines I've conjured up a naive character, a sense of menace and a rough setting.
This is because this tale has a twist based on a well known novel and if you know that book, the setting of my story would come to you at once. (Even if you didn't know the book, there is still enough in the naivety of my character for you to picture what that person would be like and get a sense of their world that way).
So it is a question then of selecting the most important thing your reader must know to make sense of your story.
Ironically, this can work well for longer pieces of fiction too even though you would have the word count to go into more detail. By focussing on what is the most important thing(s) for your readers to know, there will be no waffling and the images created in your readers' minds will be so much the stronger because of that.