I picked up a coy of Clarke's collection of shorts entitled A Meeting with Medusa recently and it contains a 180 word story about which Clarke has this to say:
Not sure I'm allowed to post the actual story (copyright and all that).
This story came about as a result of a suggestion from the late George Hay, editor and man-about-British-SF. George had the ingenious idea of putting out a complete science fiction short storyon a postcard—together with a stamp-sized photo of the author. Fans would, he believed, buy these in hundreds to mail out to their friends.
Let me tell you—it is damned hard work writing a complete SF story in 180 words. I sent the result to George Hay, and that was the last I ever heard of his scheme.
Not sure I'm allowed to post the actual story (copyright and all that).