The concept of having some kind of visual cue for a short work is an interesting concept, but there's really no established convention for that kind of thing. Perhaps some authors do this, but most don't. Nor is it consistent with most markets' guidelines on what a cover should look like. And if this was a convention, you'd run into the problem of different authors considering different lengths "long" or "short".
To me it seems like the cover should be square only if the print format is itself intended to be square. Prime examples would be comic collections and kids' books. Squares abound there.
To me it seems like the cover should be square only if the print format is itself intended to be square. Prime examples would be comic collections and kids' books. Squares abound there.