Authors, Writers, Publishers, and Book Readers
Some where in the region of 500 - 1,500 words would be a good number.
Thanks for raising the question!
I'd say more strongly 500 to 600 words. No more than that. If it is posted longer than that it seems to bore readers who view but fail to comment or pass any opinion. That's unfair if a writer has gone through the bother of posting their gems for others to read.
Short and sweet might work best.