Word length, like many other FAQ’s, does not have an easy answer. It really depends on what you’re writing, what the genre is, how the plot and pacing go, and what the story wants to be. In this digital age we have lot more options. When once novellas were shunned because publishers just didn’t take them. Now they are everywhere because we can publish on our own.
The general guidelines for lengths according to wiki:
Novel over 40,000 wordsNovella 17,500 to 40,000 wordsNovelette 7,500 to 17,500 wordsShort story under 7,500 words
(Not on wiki, but under 2k is considered flash fiction by most people.)
Another consideration is genre. In romance, 60k is normal, while in sci-fi and fantasy it’s more common to see 80-100k+ novels.
E-books have given us a lot more latitude, though it still is proven that people buy longer books more often. In that case, 80k seems to be a “magic number” in word length, as much as anything is “magic” in the publishing world.
But how long should your story be? Ask your story! Does it have a lot of twists and turns in the plot? Then go longer. Is it a simple vignette, a window into a world instead of the whole world, then go short. I’ve seen stories that are as short as 100 words that are worth a read.