You just have to be totally minimalistic in your writing style, stripping it to it's core or skeleton. But it should still read as a complete story.
Generally, the word count should not exceed 1000 words, and some say micro fiction (considering it a step further than flash fiction) should not exceed 100 words, but there seems to be no concrete 'norm' where w/c is concerned.
Here's a six word Flash by Ernest Hemingway:
"For sale: baby shoes, never worn."